The 'Two Covenant' or 'Dual Covenant' theory of salvation teaches that God established two different paths to salvation - one for by the Jews and the other for the Gentiles. The Jew's path to Him is by virtue of the Mosaic and Abrahamic Covenants - the Christians path lies through Jesus.
The Dual Covenant is said to have been developed by a man called Franz Rosenzweig who the Stanford Encyclopedia of Philosophy calls the "most original Jewish thinkers of the modern period" . In a work entitled The Star of Redemption written shortly after World War I, Franz Rosenzweig put forth put forward the idea that although a mediator is essential to the Christian, the Jew stands in no need of mediation.
Nahum Glatzer a scholar of Jewish history and philosophy quoted Rosenzweig as saying (Emphasis Added)
Christianity acknowledges the God of the Jews, not as God but as "the Father of Jesus Christ." Christianity itself cleaves to the "Lord" because it knows that the Father can be reached only through him....We are all wholly agreed as to what Christ and his church mean to the world: no one can reach the Father save through him. No one can reach the Father! But the situation is quite different for one who does not have to reach the Father because he is already with him. And this is true of the people of Israel (though not of individual Jews). 
In other words, because it is the means to the salvation of the heathen the church is extremely important however, the Jew does not have to come to the Father simply because he has been with the Father since Abraham.
Sharing The Gospel With Jews
Rosenzweig’s ideas have trickled down through the years causing even many Christian to assert that because there are two separate but equal ways to God there is no need to share the gospel with them. They were and still are the people of God, even if they do not accept Jesus Christ as their Messiah. In fact, some go as far as to consider evangelizing Jews insensitive and judgmental.
A report published by the Vatican (that does not however constitute a formal change to official Catholic doctrine) describes the fact that Jews don't need to convert as an 'unfathomable mystery'
Jews can secure eternal salvation without converting to Christianity" and that " Church should not actively seek to convert Jews to Christianity"... ""Although Jews cannot believe in Jesus Christ as the universal redeemer, they have a part in salvation, because the gifts and the calling of God are irrevocable," 
The belief that God's bond with the Jews is both unbreakable and eternal is based on their having been chosen by God ...
For you are a holy people to the Lord your God; the Lord your God has chosen you to be a people for His own possession out of all the peoples who are on the face of the earth. (Deuteronomy 7:6, 14:2 NASB)
And of course, the unconditional Abrahamic covenant.
God made three interconnected and inseparable promises to Abraham - (the third of which is i often ignored, overlooked, or spiritualized by the church) See The Promises and The Prophecies (Part II of What And Where is Heaven? ).
However, one question has to be answered.
Was There a Reason The Jews Were God's Chosen People?
God miraculously freed the Jews from bondage in Egypt, led them to the land He had promised Abraham so many years earlier and specifically told them that they were His chosen people. He also entrusted them with His laws and commandments.
The million dollar question is why?
Why were the Jews singled out from all other people on the earth? In fact, the Father didn't just choose a nation of people but literally created one when He called Abraham out from Ur and made him a very solemn promise
And I will make you a great nation, And I will bless you, And make your name great; And so you shall be a blessing; And I will bless those who bless you, And the one who curses you I will curse. And in you all the families of the earth will be blessed." (Genesis 12:2-3 NASB)
This promise was repeated to Abraham's son and grandson Isaac and Jacob and, as we know, it was Jacob whose twelve sons became the fathers of the 12 tribes of Israel.
In other words, God purposed to bless the whole world ("all the families of the earth") through this one man and the nation descended from him. Although the return of the Jews and the establishing of Israel as a sovereign and independent nation was a unprecedented modern miracle the whole world has never been blessed by them in any way other than through Christ.
The Savior had to be born into the human race which is why God created a very small nation called Israel that he nurtured and preserved. They were entrusted with God's laws and, above all, chosen to birth the Messiah who would be the Savior of the world thus fulfilling God's promise to Abraham.
In other words, God plan of salvation was set in motion almost from day one.
All of which brings up one other question ...
Does Being God's Chosen Nation Mean That The Jews Are Automatically Saved?
The answer to that question is an unqualified no!
The fact that they were given God's laws never meant that they were exempt from keeping those laws. Much to the contrary they were held to a very high standard. For example, even before they set foot in the promised land Israel was given the commandments the Father expected them to live by. If they kept those commandments they would prosper and multiply - if not, it would go very badly for them (See Deuteronomy 30).
Punishment for breaking the law was severe. Romans 6:23 clearly states that the consequences of sin is death however, there was forgiveness for sins as Leviticus 17:11 says and Hebrews 9:22 reiterates
'For the life of the flesh is in the blood, and I have given it to you on the altar to make atonement for your souls; for it is the blood by reason of the life that makes atonement.' (Leviticus 17:11 NASB)
"..all things are cleansed with blood, and without shedding of blood there is no forgiveness."
In the animal sacrifices of the Old Testament, the animal functioned as a substitute for the offender and bore the punishment of the person who had sinned. Although there were quite a few prescribed sacrifices for various sins, The Jewish feast of Yom Kippur (Day of Atonement) observed once a year was the culmination of the Ten Days of Penitence that began with Rosh Hashanah. On this, the most solemn and important holy day on the Jewish calendar, the High Priest would enter the Temple's inner sanctum - the Holy of Holies - and sprinkle the blood of the sacrifice on the mercy seat. Through this yearly act, atonement was made for the sins of all Israel and brought reconciliation between the people and God. (Leviticus 16:30).
According to the sixteenth chapter of Leviticus, Yom Kippur was as day of a solemn fast, on which no food or drink could be consumed, and on which all work was forbidden. The people prepared themselves, with fasting and prayer, to stand trial before the heavenly court that would review their life of the past year, and render a verdict. The solemnity of the occasion is highlighted by the fact that God said that anyone who did not "afflict his soul" on that day would be cut off from the community.
The problem is the temple was destroyed in AD 70 which means that without the Temple and the animal sacrifices made therein (especially the ones made on Yom Kippur) there hasn't been any atonement for sin for the Jews for near 2,000 years.
How can there be a true Judaism with no substitute atonement as Moses directed in the Torah? All the detailed instructions as to the sacrifices and temple observances came straight from God, yet the fact that God allowed the Temple to be destroyed doesn't seem to set off very many alarm bells.
This belief that a person can obtain forgiveness without from a blood sacrifice contradicts the Torah, that clearly states that blood on the altar was the means by which atonement was made. There was no other way. Let me quote Leviticus 17:11 once more...
'For the life of the flesh is in the blood, and I have given it to you on the altar to make atonement for your souls; for it is the blood by reason of the life that makes atonement.' (Leviticus 17:11 NASB)
In fact, modern Judaism would have us believe that God allowed changes made based on decisions made by Rabbis.
See Judaism And The Atonement
So why did God allow the Temple to be destroyed?
Simply because because the Levitical system was only meant to prefigure Christ's one time sacrifice for sin. After He died there was no longer any need of the Old Testament sanctuary and its rituals. Hebrews 9 and 10 compare the temporary nature of the Old Testament sacrifices with the permanence of Christ's sacrifice in the New Testament. Once the permanent was established the temporary was done away with. As Jesus said,
"I am the way and the truth and the life. No one comes to the Father except through me" (John 14:6).
"No one" means no one - not one single person.
For a Jew to be saved, he or she must come to God the Father through faith in Jesus the Messiah. Those who reject Jesus' sacrifice on the cross will find that "no sacrifice for sins is left, but only a fearful expectation of judgment ...." (Hebrews 10:26–27).
Those who believe that the Jews have no need of the Gospel need to explain why in the New Testament...
The Gospel Was Taken to The Jews First - Then To The Gentiles
Jesus' ministry was to the Jews.
Jesus was going through all the cities and villages, teaching in their synagogues and proclaiming the gospel of the kingdom, and healing every kind of disease and every kind of sickness. (Matthew 9:35 NASB) Also See Matthew 4:23–25.
Only on a rare occasion did He minister to the Gentiles. Examples include the Syrophoenician woman in Mark 7:24 on and the Roman centurion's servant in Matthew 8:5 on. In fact, when Jesus sent out the twelve apostles in (Matthew 10:5-6), He told them not to go in the way of the Gentiles, nor enter any city of the Samaritans; but to go instead to the lost sheep of the house of Israel.
Then after He was resurrected, He told the disciples
"Thus it is written, that the Christ would suffer and rise again from the dead the third day, and that repentance for forgiveness of sins would be proclaimed in His name to all the nations, beginning from Jerusalem. "You are witnesses of these things. (Luke 24:46-48 NASB)
but you will receive power when the Holy Spirit has come upon you; and you shall be My witnesses both in Jerusalem, and in all Judea and Samaria, and even to the remotest part of the earth." (Acts 1:8 NASB)
In his very first public message, Peter preached the Gospel message to the multitudes from many nations gathered in Jerusalem for the Feast of Pentecost. The Bible describes the crowd as "Jews living in Jerusalem, devout men from every nation under heaven" (Acts 2:5)
It is quite obvious that Peter's message was specifically aimed at the Jews when he said "Men of Judea and all you who live in Jerusalem, let this be known to you and give heed to my words." In verse 22 he again reiterated who he was speaking to ""Men of Israel, listen to these words" This was followed by
"Therefore let all the house of Israel know for certain that God has made Him both Lord and Christ - this Jesus whom you crucified." (Acts 2:36 NASB)
Not until we get through the first nine chapters of Acts do we find the Gospel being taken to the Gentiles - and that happened only because Cornelius a Roman centurion. was miraculously directed to send for Peter, who was also miraculously prepared to answer the summons. Peter traveled to Caesarea, preached the gospel to Cornelius and his friends, and saw the miraculous gifts of the Spirit poured upon the Gentiles.
However, the apostles and brethren in Jerusalem took issue with the fact that Peter had gone to uncircumcised men until they heard the circumstances surrounding Peter and Cornelius' meeting. In Peter's words
After there had been much debate, Peter stood up and said to them, "Brethren, you know that in the early days God made a choice among you, that by my mouth the Gentiles would hear the word of the gospel and believe. "And God, who knows the heart, testified to them giving them the Holy Spirit, just as He also did to us; and He made no distinction between us and them, cleansing their hearts by faith. (Acts 15:7-9 NASB)
It was only then that the other apostles realized that the Gospel was also intended for the Gentiles. They then determined that the Gentiles did not have to live by their rules (ex. circumcision) except they needed to abstain from things sacrificed to idols, from blood, from things strangled and from fornication. (Acts 15:28-29)
The belief that the Jews do not need to hear the Gospel because they already have their own path to the Father does not make any sense in light of several statements Paul made in his letter to the church in Rome. His very first words make it very clear that the gospel of Jesus Christ is for everyone
For I am not ashamed of the gospel, for it is the power of God for salvation to everyone who believes, to the Jew first (Gk. proton) and also to the Greek. (Romans 1:16 NASB)
The Greek word proton means first in time, place or rank as the following examples show.
Now after He had risen early on the first day of the week, He first (Gk. proton) appeared to Mary Magdalene, from whom He had cast out seven demons. (Mark 16:9 NASB)
Paul and Barnabas spoke out boldly and said, "It was necessary that the word of God be spoken to you first (Gk. proton) ; since you repudiate it and judge yourselves unworthy of eternal life, behold, we are turning to the Gentiles. (Acts 13:46 NASB)
For the Lord Himself will descend from heaven with a shout, with the voice of the archangel and with the trumpet of God, and the dead in Christ will rise first (Gk. proton) . (1 Thessalonians 4:16 NASB)
For I am mindful of the sincere faith within you, which first (Gk. proton) dwelt in your grandmother Lois and your mother Eunice, and I am sure that it is in you as well. (2 Timothy 1:5 NASB)
Note: When Paul said he was not "ashamed", he probably had Jesus' Luke 9:26 in mind - "For whoever is ashamed of Me and My words, the Son of Man will be ashamed of him when He comes in His glory..." Also See 2 Timothy 1:7–8. Similarly we are not to be ashamed to share the good news of the gospel with anyone.
There are countless examples of Paul, the "apostle to the Gentiles" preaching the message to the Jews. In fact, his normal pattern was to go to them first before the Gentiles. As Paul and Barnabas said to some disbelieving Jews in Antioch
"It was necessary that the word of God be spoken to you first (Gk. proton); since you repudiate it and judge yourselves unworthy of eternal life, behold, we are turning to the Gentiles. (Acts 13:46 NASB)
Other examples include,
and immediately he began to proclaim Jesus in the synagogues, saying, "He is the Son of God." (Acts 9:20 NASB)
And he was talking and arguing with the Hellenistic Jews; but they were attempting to put him to death. (Acts 9:29 NASB)
In Iconium they entered the synagogue of the Jews together, and spoke in such a manner that a large number of people believed, both of Jews and of Greeks. (Acts 14:1 NASB)
when they had traveled through Amphipolis and Apollonia, they came to Thessalonica, where there was a synagogue of the Jews. And according to Paul's custom, he went to them, and for three Sabbaths reasoned with them from the Scriptures, (Acts 17:1-2 NASB)
So he was reasoning in the synagogue with the Jews and the God-fearing Gentiles, and in the market place every day with those who happened to be present. (Acts 17:17 NASB)
They came to Ephesus, and he left them there. Now he himself entered the synagogue and reasoned with the Jews(Acts 18:19 NASB)
In the second chapter of Romans Paul contrasted Abraham's blood descendants with those who have faith, regardless of their heritage:
"A person is not a Jew who is one only outwardly, nor is circumcision merely outward and physical. No, a person is a Jew who is one inwardly; and circumcision is circumcision of the heart, by the Spirit, not by the written code" (Romans 2:28–29).
In fact, Paul felt so strongly for the Jews who had condemned themselves by rejecting the Gospel message that he said he had "... great sorrow and unceasing grief" in his heart. Paul even wished he could be accursed and give up his own salvation for the sake of his brethren...
For I could wish that I myself were accursed, separated from Christ for the sake of my brethren, my kinsmen according to the flesh, who are Israelites, to whom belongs the adoption as sons, and the glory and the covenants and the giving of the Law and the temple service and the promises, (Romans 9:2-4 NASB)
His grief was caused by the fact that, as he went on to explain,
But it is not as though the word of God has failed. For they are not all Israel who are descended from Israel; nor are they all children because they are Abraham's descendants, but: "through Isaac your descendants will be named." That is, it is not the children of the flesh who are children of God, but the children of the promise are regarded as descendants. (Romans 9:68 NASB)
That faith in Christ makes one Abraham's spiritual descendent is emphasized when in Jesus' time Zacchaeus, the chief tax collector and a very rich man, repented and resolved to pay back all he had defrauded anyone of. Jesus told him, "Today salvation has come to this house, because he, too, is a son of Abraham (Luke 19:9), the father of all who have faith (Romans 4:11).
Then just a few verses later, Paul expressed a heartfelt wish...
Brethren, my heart's desire and my prayer to God for them is for their salvation. For I testify about them that they have a zeal for God, but not in accordance with knowledge. (Romans 10:1-2 NASB)
John the Baptist warned the crowds who went to be baptized by him against trusting that their lineage made them right with God. He called them "brood of vipers' and asked who warned them to flee from the wrath to come? He went on to say
"Therefore bear fruits in keeping with repentance, and do not begin to say to yourselves, 'We have Abraham for our father,' for I say to you that from these stones God is able to raise up children to Abraham. "Indeed the axe is already laid at the root of the trees; so every tree that does not bear good fruit is cut down and thrown into the fire." (Luke 3:8-9 NASB)
Jesus: In an epilogue to the parable of the man and his two sons - and the vineyard let out to husbandmen (a sort of a tenant farmer), Jesus told the chief priests and the elders
Therefore I say to you, the kingdom of God will be taken away from you and given to a people, producing the fruit of it. (Matthew 21:43 NASB)
Large Numbers of Jew Believed After Hearing The Gospel Preached By The Apostles
Three Thousand on Pentecost - "... those who had received his word were baptized; and that day there were added about three thousand souls. (Acts 2:41 NASB)
Five Thousand: But many of those who had heard the message believed; and the number of the men came to be about five thousand. (Acts 4:4 NASB)
Multitudes: And all the more believers in the Lord, multitudes of men and women, were constantly added to their number, (Acts 5:14 NASB)
Some Priests: The word of God kept on spreading; and the number of the disciples continued to increase greatly in Jerusalem, and a great many of the priests were becoming obedient to the faith. (Acts 6:7 NASB)
Thousands: And when they heard it they began glorifying God; and they said to him, "You see, brother, how many thousands there are among the Jews of those who have believed, and they are all zealous for the Law; (Acts 21:20 NASB)
In view of the New Testament Apostles's untiring efforts to take the message of salvation to the Jews I have one question...
When Was The Call to Take The Gospel To The Jews Revoked?
Do Jews Have To Abandon Their Heritage In Order to be Saved?
Just as at the Jerusalem Council when the elders determined that the Gentiles did not need to become Jews in order to be saved. Jews do not need to become Gentiles and forsake their heritage and practices in order to be saved.
In fact, at some point in the early church people had begun to heed the rumor that Paul was teaching the Jews to forsake Moses, telling them not to circumcise their children nor to walk according to the customs. (Acts 21:21 NASB)
The elders instructed Paul to publicly demonstrate that he was not opposed to the Jews continuing to keep certain customs, although the Gentiles who came to Jesus were not required to do so.
Paul showed that he considered the law of Moses holy by purifying himself along with four Christian Jews who were dedicating themselves to the Lord by observing the vow of the Nazarite. During this period they had to let their hair grow, abstain from wine, strong drink, vinegar, grapes, and not go near a dead body even if it was a near relative.
They were to also to present an offering when the days of the vow were completed (Numbers 6) which Paul paid for in the case of the four men. (Acts 21:20-26).
Our Debt To The Jews
Sadly, the fact that Christians owe a great debt to the Jews seems to have been largely forgotten. Had it not been for them and the awe-inspiring faithfulness with which they clung to the God who chose them, there would have been no law, no Scriptures, no Savior and no salvation.
We seem to gloss over the fact that it is we who have been grafted into their tree, not the other was around.
But if some of the branches were broken off, and you, being a wild olive, were grafted in among them and became partaker with them of the rich root of the olive tree, do not be arrogant toward the branches; but if you are arrogant, remember that it is not you who supports the root, but the root supports you. (Romans 11:17-18 NASB)
Certainly the chapter goes on to say that being of Jewish descent does not guarantee a place in the kingdom - only faith does. However, God never forgets His promises even if man does not pay them any heed. God has not told us how it will come about but we can take confidence in the words of the prophet Zechariah.
I will pour out on the house of David and on the inhabitants of Jerusalem, the Spirit of grace and of supplication, so that they will look on Me whom they have pierced; and they will mourn for Him, as one mourns for an only son, and they will weep bitterly over Him like the bitter weeping over a firstborn. (Zechariah 12:10 NASB)
Paul echoed the thought that God has not totally rejected the Jews and still has plans for them,
For I do not want you, brethren, to be uninformed of this mystery - so that you will not be wise in your own estimation - that a partial hardening has happened to Israel until the fullness of the Gentiles has come in; and so all Israel will be saved; just as it is written, "The Deliverer will come from Zion, He will remove ungodliness from Jacob." "This is My covenant with them, when I take away their sins." From the standpoint of the gospel they are enemies for your sake, but from the standpoint of God's choice they are beloved for the sake of the fathers; for the gifts and the calling of God are irrevocable. (Romans 11:25-29 NASB)
And all we can do is preach the Gospel and pray that the person to whom we are speaking will realize that there are some things he, or she has missed.
 Stanford Encyclopedia of Philosophy. Franz Rosenzweig https://plato.stanford.edu/entries/rosenzweig/
 Franz Rosenzweig: His Life and Thought. Hackett Publishing Company, Inc.; Third Edition, 3 (June 1, 1998) Pg. 341
 Vatican: Jews don't need Christ to be saved. Israel National News https://www.israelnationalnews.com/news/204717