Ecumenism: The word ecumenism comes from a family of Classical Greek words: oikos, meaning a house, family, people, or nation;
Oikoumenē- “the whole inhabited world”; and oikoumenikos, “open to or participating in the whole world.” For example, Oikoumenē describes
The place of God’s reconciling mission - This gospel of the kingdom shall be preached in the whole world as a testimony to all the nations, and then the end will come. (Matthew 24:14 NASB)
The unity of the Roman Empire - Now in those days a decree went out from Caesar Augustus, that a census be taken of all the inhabited earth. (Luke 2:1 NASB)
The kingdoms of the earth - And he led Him up and showed Him all the kingdoms of the world in a moment of time. (Luke 4:5 NASB)
In later centuries the word ecumenical was used to denote church councils (e.g., Nicaea, Ephesus etc.) whose decisions represented the universal church. More recently ecumenism has meant either an organized attempt to bring about cooperation and unity among Christians of all denominations or even worldwide cooperation and unity among all religions.
As said by Dr. David R. Reagan of Lamb Lion Ministries
The most popular apostasy in Christendom today is the teaching that God has revealed Himself in many different ways to different cultures and that, therefore, all religions worship the same god, but just use different names. From this viewpoint, the Allah of Islam is the same as the Yahweh of Judaism and both are the same as the Krishna of Hinduism. The natural conclusion that is drawn from this apostate idea is that there are many different paths to God, Jesus being only one of them. This has led liberal leaders of groups like The National Council of Churches in the United States and the World Council to condemn missionary activity as "arrogant" and "anti-cultural." 
See Religious Pluralism
It is tragically true that few of those who believe 'all spiritual beliefs are valid paths to God" seem to have made an in depth study of various religions to see if their claims are based on fact, or fairy dust
There is no question that unity is important - However, most of the preaching abut unity assumes that there is no truth that people are required to stand on and defend - that how we interpret Biblical precepts is our "opinion" and it is wrong to let our "opinions" divide us.
Unity is an agreement of sorts - a oneness of mind and purpose. As Amos once wrote "Can two walk together, except they be agreed?" The fact of the matter is that God never intended us to be unified with everyone regardless of whether we happen to take the same position on some social issues - the gospel has to be our top priority. Quite simply we cannot be agreed with everyone. See True and False Unity
Parliament of the World’s Religions
Interreligious Dialogue or New Age Rally? Held in Chicago in 1993, exactly a century after the watershed gathering held there in 1893, the Parliament of the World's Religions brought together swamis, sadhus, rinpoches, monks, arhats, avatars, priests, priestesses, ministers, witches, shamans, saints, and seekers. A kaleidoscope of religious worthies upon which the watchful gods and muses on the ceiling beamed approvingly. However many prominent New Agers were involved at different levels with the organization of the event which gives rise to the question... Did promotion of the New Age agenda underlie the Parliament's ostensible purpose of furthering interreligious dialogue?
The United Religions Initiative
You’re familiar with the old saying, “The road to hell is paved with good intentions.” Thanks to Bishop William Swing of the California Diocese of the Episcopal Church, that road is not only paved, but it’s being widened and the lines are being repainted. In other words, the URI is designed to make a long-term impact on the global religious scene.
World Council Of Churches
Founded in 1948, the World Council of churches represents more than half a billion Protestant and Orthodox Church members from 350 denominations in over 100 countries. They claim to be "a fellowship of Churches which accept Jesus Christ our Lord as God and Saviour" with the mission of "promoting justice and peace in our world as an expression of faith in the Triune God". However, with their involvement in goddess worship, paganism, ecumenism and every other conceivable form of deviancy, The WCC is one of the worst of the apostate bodies claiming to be Christian.
William Franklin Graham, Jr. was, arguably, the best known, best loved, and most influential evangelical during the approximately 50 years he was in the public forum. However, many have rightly pointed to him as the father of modern ecumenism. Billy Graham put the stamp of approval on the false gospel preached by the Roman Catholic church and has undoubtedly caused many to think that Roman Catholicism must be true Christianity. This is not the only area where the Rev. Graham has compromised the Gospel.
Holding Hands With The Pope
The idea that we no longer need oppose the heresies of Rome because they are not really "heresies" is catching on fast. Leaders in both camps are jointly signing ecumenical documents representing a combined effort to "bury the hatchet" and work together as "teammates" instead of antagonists. While it all sounds very good - even very ‘Christian’ to the undiscerning, this also means that serious doctrinal differences have begun to be down played and that clear biblical principles are being blatantly ignored by those who are promoting more rapport with Roman Catholicism.
The Two Faces of Catholicism
On the one hand the Vatican has made a concerted effort since 1965 to bring their "separated brothers back home to "holy mother the church." On the other hand the Catholic church fraternizes .. joins in prayer with ... and actively endorses other religions.. Pope John Paul II's bizarre involvement with and approval of other anti-Christian religions has caused even many Catholics to call Pope John Paul II the Anti Pope.... However many Protestants see this as a much more sinister sign.
The Scriptural View
The Inclusiveness of Jesus
It’s true that the inclusiveness of Jesus was extraordinary. Unlike his religious contemporaries, Jesus included among his followers those who were generally excluded from religious life, if not polite society, people such as tax-collectors, “sinners,” lepers, and women. Yet, the inclusiveness of Jesus was not of the “come as you are” sort. Jesus offered new, transformed life in the kingdom of God, not acceptance of all people as they were in their broken, sinful state.
Should Christians Join In Interfaith Communion?
There is great social pressure today to conform to whatever cultural liberals decide is "politically correct"
True and False Unity
The idea of unity that many Christian leaders preach and teach is not true unity.
 Â Dr. David R. Reagan. The One World Religion. Lamb & Lion Ministries. https://christinprophecy.org/articles/the-one-world-religion/