Separation of Church and Church

The Christian circles in which I grew up taught that fellowship between denominations was a dangerous thing. I was taught that the true church must separate itself from every other denomination that does not teach or believe exactly the same things that we do. According to this teaching the greatest evil in the church is the ecumenical movement, desiring to see the different church groups associate together and have a joint ministry in the world. I have since come to the conclusion that such standards of separation are unbiblical.
Yet, there is in the Bible clear lines that must be drawn for separation from others who profess Christianity. We should not separate from other Christians simply because they do not belong to the same church or denomination. There are other, more telling and sinister reasons to separate from professing Christians. One of many passages that provide clues to the basis for separation is the warning John gave to the church in 1 John 2:18-24, "Children, it is the last hour; and just as you heard that antichrist is coming, even now many antichrists have appeared; from this we know that it is the last hour. They went out from us, but they were not really of us; for if they had been of us, they would have remained with us; but they went out, so that it would be shown that they all are not of us. But you have an anointing from the Holy One, and you all know. I have not written to you because you do not know the truth, but because you do know it, and because no lie is of the truth. Who is the liar but the one who denies that Jesus is the Christ? This is the antichrist, the one who denies the Father and the Son. Whoever denies the Son does not have the Father; the one who confesses the Son has the Father also. As for you, let that abide in you which you heard from the beginning. If what you heard from the beginning abides in you, you also will abide in the Son and in the Father."
John's warning is very clear, there are those who profess to be followers of Jesus Christ that deny the biblical truths about Jesus Christ. These people are called antichrists and their purpose is to deceive others and lead them away from the truth.
John's admonition in this passage is to cling to that which we know is true. The first century church could honestly adhere to those things which the apostles taught them as being absolute truth. The best way for us to know absolute truth is to study God's Word. Rather than relying on the pastor or teacher to tell us what to believe, we must search it out for ourselves. Then, when we are faced with professing believers who deny the biblical truths about Jesus, we will understand that they are not truly followers of Christ. These are the ones with whom we are to avoid fellowship. In 2 John we are told not to give them a greeting or invite them into our place of worship because they will try to lead astray weak and carnal Christians.
Along these lines, John later teaches in 1 John 4 to test the spirits to see if they are from God. Many teachers claim to believe in Jesus yet they deny all that the Bible claims to be true of him (such as His relationship to the Father and His deity). It is important to test them because we might be fellowshipping with someone who is an enemy of God.
It is vital for our relationship with God that we abide (make our dwelling place) in the truth about His Son. The Jesus revealed in the Bible is the only way to the Father. The Jesus that the antichrists teach is not the true Christ and cannot save anyone. As we pursue intimacy with God, we must cast off anything, including associations, that hinder the growth of that relationship.