Also See Is The Sinner’s Prayer Effective?
There is no more fundamental, vital question that anyone can ask than 'what must I do to be saved?’ - Unfortunately, there are no end of well meaning Christians who, in answer to that life and death question, will inform you that You have to ask Jesus into your heart. In fact, you will find this phrase liberally sprinkled through the Christian world... in Gospel tracts, preached from pulpits and taught by evangelists. While it is undoubtedly well meaning, and well intended, The burning question is whether this phrase accurately communicates the truth of Scripture. Several questions need to be asked, and answered...
1. Is it Biblically accurate or just meaningless jargon or a confusing cliché?
2. Is it (like the prayer of salvation) based on emotion, with little understanding of what is really involved.
3. Is someone who has simply asked Jesus into their heart saved?
4. Does the whole issue boil down to a matter of mere semantics?
The answers to these questions provide some very reasons as to why one should never use this man-made phrase.
A Very Confusing Cliché
Although the expression is common enough, even after close to thirty years of being a Christian I am still unsure of what it is supposed to mean. If I am still puzzled by this cliché, I can only imagine the effect it has on a non-Christian adult, leave alone a child … any child.
One can only imagine what a non believer must think when exhorted to ask Jesus into their hearts?. They must wonder how this works - whether it is literal, or some kind of metaphysical/ supernatural experience? Is it symbolic? If so.. What is it supposed to mean, and what is it supposed to do? The confusion is completely understandable considering no good clarification of this phrase actually exists. Is it any wonder that so often presented with completely meaningless phrases that communicate nothing intelligible, much of the world dismisses us as a credulous superstitious bunch?
The situation is probably made worse with children who usually think in very literal terms thus probably imagine a literal Jesus literally living inside their hearts. Lower ventricle anyone?
Additionally, the phrase
Is NOT Found Anywhere in The Bible.
If you had never ever read a Christian book or tract, talked to another Christian, or attended a Christian service or evangelistic crusade, but had simply read the Bible from cover to cover half a dozen times, you would never have known that you had to ask Jesus into your heart to be saved.
Because it simply isn't there.
The book of Acts tells us of the spread of the very early church and exactly how and what people did to be saved and become Christians. Yet there is not one single instance of anyone asking Jesus into their heart. Jesus never said it, the apostles never said it. In fact, nowhere in the Scriptures is there even one example of any individuals ever asking Jesus into their heart. So if it is never found in the Bible, why use it?
It Is Not How One Is Saved.
When the Philippian jailer asked Paul and Silas the million dollar question "Sirs, what must I do to be saved?", they did not tell him to ask Jesus into his heart. Nor did they tell him to recite the sinners prayer, read the Four Spiritual Laws nor any of our other inventions. Their answer was simple and concise - "Believe on the Lord Jesus Christ and thou shalt be saved and thy house." (Acts 16:30-31)
Here are a few examples of what the Bible unambiguously teaches (All Emphasis Added)
For God so loved the world, that He gave His only begotten Son, that whoever believes in Him shall not perish, but have eternal life. (John 3:16 NASB)
"He who believes in the Son has eternal life; but he who does not obey the Son will not see life, but the wrath of God abides on him." (John 3:36 NASB)
"Truly, truly, I say to you, he who hears My word, and believes Him who sent Me, has eternal life, and does not come into judgment, but has passed out of death into life. (John 5:24 NASB)
"Therefore I said to you that you will die in your sins; for unless you believe that I am He, you will die in your sins." (John 8:24 NASB)
It Bypasses A True Understanding Of The Gospel.
The biggest problem is not that we may phrase something a little differently but that this ill-defined cliché misses the point entirely. The Gospel of Jesus Christ is the good news of who Jesus Christ is and what He has done to save lost sinners from the just penalty for their sins. That God releases us from the demands of eternal justice on the basis that the Christ paid our fine. For an In Depth Understanding of The Salvation God has Offered Us. See Salvation
As believers we are also instructed to proclaim the Gospel to others...
And He said to them, "Go into all the world and preach the gospel to all creation. (Mark 16:15 NASB)
and that repentance for forgiveness of sins would be proclaimed in His name to all the nations, beginning from Jerusalem. (Luke 24:47 NASB)
namely, that God was in Christ reconciling the world to Himself, not counting their trespasses against them, and He has committed to us the word of reconciliation. (2 Corinthians 5:19 NASB)
Obscuring the primary issue is not how to do so.
Bob Wilkin, of the Grace Evangelical Society, testifies of the far reaching implications this inaccuracy can have.
One day while randomly walking up to people sitting on a pretty hillside on my Southern California campus, I found a fellow student who was open to spiritual things. I shared the gospel with him and asked him to invite Jesus into his heart. I was delighted when he did just that.
A week later on our follow-up appointment, my joy turned to chagrin. He announced to me that upon reading the literature I had given him to study he had discovered that I was saying that Jesus Christ was the only way to God. He vehemently disagreed with that. He believed that Jesus was one of the prophets who pointed to God. He wanted all of the prophets in his heart, so on that basis he invited Jesus in.
It turned out that he was a follower of an Eastern religious sect. My ask-Jesus-in appeal clearly didn't work with him. He asked Jesus in sincerely, yet without trusting in Him alone for eternal life.
Shortly after the second of the above incidents occurred I stopped challenging people to ask Jesus in. I began instead to ask people to trust in Christ alone.
Years later I was teaching an evangelism course at a Bible College in East Texas. I had my students write out their testimonies after I had explained what I have recounted above.
I found that quite a few of the students went through years of confusion because someone told them as children that if they asked Jesus into their hearts they would be saved. They wondered if they had done it right. They wondered if they had been sincere enough. So they asked Him in over and over again for years. They couldn't gain assurance. Finally someone shared with them that to be saved they had to trust in Christ alone. Only then, by their own testimony, did they come to faith in Christ. Years of inviting Him into their lives had only confused and frustrated them.
I had one student whose testimony ended with him saying that he invited Jesus into His heart for eternal life. In conversation with him I learned that he had come to faith in Christ alone in my class that semester and that when he wrote his testimony he just wrote up what he had always written before. I explained to him that if he had just come to faith in Christ alone that semester, then he needed to rewrite his testimony. He lit up as it dawned on him that faith in Christ, not inviting Jesus in, was the real issue. 
In other words, anyone - young or old - can ask Jesus into their hearts without a basic understanding of the Gospel message - without knowing a thing about sin and its consequences, that Jesus who is God Almighty, took on human form and came to earth to die on the cross by which He paid for that sin, the reality of Christ's resurrection, and that they cannot live the rest of their lives however they please and still be welcome in God's presence. See The Myth of Faith Alone
Even followers of another religion may be willing to ask Jesus into their hearts as many of them believe that He was a very wise man and a great teacher. They may not realize this involves believing He is the only way to salvation and forsaking all other 'gods' and following Him come what may.
When speaking to a non believer the only issues are sin and the need for forgiveness and that no one is forgiven without repentance - an indispensable part of the Gospel. Any so called Gospel message that does not include the need for true Biblical repentance cannot be counted as the gospel at all. It is insufficient to save.
[See How The Bible Defines Sin and Repentance]
How exactly then can this popular phrase be construed as the Gospel?
(Perhaps the best way to realize someone's understanding of the Gospel message is by asking them the simple question.. "If you were to die tonight and God were to ask you, 'Why should I let you into my heaven?' What would you say?". The answer to that question will tell you whether they are on track or not.)
It Could Create a False Assurance of Salvation
It is to be again emphasized that there is NO instance of anyone in Scripture being saved by asking Jesus into their hearts.
It is however, completely possible that someone may fully understand the Gospel and trust Jesus as their Saviour when they ask Him into their hearts, in which case there is little doubt that they are saved not because of this confusing cliché but because they have been reconciled to God through their faith in Christ.
However, there is still no excuse for Biblical inaccuracy.
But the other side of the coin is very worrisome.. that someone may NOT have a clear view of the Gospel and may be relying on an ambiguous, imprecise instruction to save them for all eternity.
The Counter Arguments
So Many People and Ministries Use the Phrase
If the accuracy of what we believe is determined by sheer numbers, we would all be Catholics or Muslims. The issue is not how many people or even which people believe a certain thing, but whether it Biblical or not. And in this case the burning question is whether this phrase accurately communicates the truth of Scripture.
Don't Romans 10:9, Revelation 3:20, and Ephesians 3:17, Support Asking Jesus Into Ones Heart?
The simple answer is.. No! They do not
'Behold, I stand at the door and knock; if anyone hears My voice and opens the door, I will come in to him and will dine with him, and he with Me. (Revelation 3:20 NASB)
It is quite amazing to what extent this verse has been misused and misinterpreted. We think something along the lines of 'Aw, poor Jesus is out there is the cold, shivering, waiting for someone to let Him in. Wont you go ahead and let poor Jesus into your heart?" Except the text never states that Jesus is seeking entry to your heart.
Much to the contrary, our Lord is NOT knocking on the door of the unsaved, but on the door of the church in Laodicea (See Revelation 3:14). While it is absolutely appalling that Jesus has to stand outside of His own church, knocking for them to let Him in, the real tragedy is how often this situation is being repeated across this land.
Using Revelation 3:20 to support this cliché is superimposing a meaning on the text that simply does not exist.
that if you confess with your mouth Jesus as Lord, and believe in your heart that God raised Him from the dead, you will be saved;
This was addressed primarily to the Jews who believed in God, but who fell short of believing that God raised Jesus from the dead, which is the cornerstone of salvation..
" believe in your heart that God raised Him from the dead" simply means that if a person sincerely and truly believe this, so that the external profession of faith corresponds with real internal feelings.. the person shall be saved.
so that Christ may dwell in your hearts through faith; and that you, being rooted and grounded in love,
Note that this is not an evangelistic verse - it refers to Christ dwelling in the hearts of the Ephesians who were already believers. Expressions like this are fairly commonplace in the New Testament, where God is said to dwell in us and we are said to be the temples of the Holy Spirit.
Jesus answered and said to him, "If anyone loves Me, he will keep My word; and My Father will love him, and We will come to him and make Our abode with him. (John 14:23 NASB)
Do you not know that you are a temple of God and that the Spirit of God dwells in you? (1 Corinthians 3:16 NASB)
Or do you not know that your body is a temple of the Holy Spirit who is in you, whom you have from God, and that you are not your own? (1 Corinthians 6:19 NASB)
To be noted is that the Bible also speaks of sin dwelling in a person which is a figurative expressions implying that the person is strongly influenced by sin.
So now, no longer am I the one doing it, but sin which dwells in me. (Romans 7:17 NASB)
In contrast when it is said that "the Spirit of God dwells" in the Christian, it simply means he is under the influence of God with the views, feelings, and moral conduct etc. of someone who is obeying the command to "be Holy" and whom God is pleased with - NOT just anyone who has done nothing more than say some words.
See What is Holiness?
Ephesians 3:17 happens to be the one place where the Bible specifically refers to Jesus dwelling in a person's heart. However, the Bible makes it very clear that there is only one Spirit.
There is one body and one Spirit, just as also you were called in one hope of your calling; (Ephesians 4:4 NASB)
Now there are varieties of gifts, but the same Spirit. (1 Corinthians 12:4 NASB)
For by one Spirit we were all baptized into one body, whether Jews or Greeks, whether slaves or free, and we were all made to drink of one Spirit. (1 Corinthians 12:13 NASB)
There is no question that the Spirit of God, the Spirit of Christ and the Holy Spirit are all the same Spirit. Thus there is absolutely no difference between Ephesians 3:17, 1 Corinthians 3:16 and 1 Corinthians 6:19.
See The Holy Spirit ... a Separate Person or The Divine Presence And Power Of The Father Himself?
Isn't This All Simply A Matter Of Semantics?
The single most consequential decision a person will ever make is to determine where they will spend eternity yet, when it comes to pointing a person towards eternal salvation, we tolerate biblical inaccuracy and doctrinal fuzziness.
As has been shown above trusting in Jesus Christ alone and asking Jesus into your heart are not necessarily saying the same thing.
Those who believe in the inerrancy of the Bible ought especially to be concerned with accuracy in communicating the truth. Words are the means God chose to tell us the good news, and words are the means we employ to explain the Gospel to others. Therefore "a correct choice of words is important, even essential, in stating the Gospel well." 
In fact, a fuzzy or inaccurate Gospel message could very well be construed as 'another Gospel;' about which Paul had this to say,
But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed. As we said before, so say I now again, if any man preach any other gospel unto you than that ye have received, let him be accursed. (Gal. 1:8-9)
There is an interesting verse in 1 Corinthians, which, although directly referring to the interpretation of tongues, is perfectly illustrative of the principle,
For if the trumpet give an uncertain sound, who shall prepare himself to the battle? (1 Corinthians 14:8)
The battle is on for the souls of men. If the sound of the trumpet is garbled and not understood by the army, the soldiers cannot know what is required of them.
 Bob Wilkin. Don't Ask. https://faithalone.org/magazine/y1991/91aug2.html
 Charles Ryrie. So Great Salvation. p. 24