II Corinthians 6:14, 17 "Be ye not unequally yoked together with unbelievers. Wherefore come out from among them, and be ye separate."
Separation of light from darkness is a command of God from the very beginning of Scripture. God divided the light from the darkness. The very word "separate" in II Corinthians 6:17 is the Greek word "aphorize", from which we get our word horizon. That's where darkness and light are divided. It means literally to be marked off by boundaries.
The very word "church" means a called out body. We are called to be separate, marked off as different from the world.
We are commanded to separate from unbelievers to come out from among them. We are not to be unequally yoked together with them. We must not marry an unbeliever....to do so is to be unequally yoked. We must not link up in spiritual endeavors with those who are not saved. To do so sends a mixed signal of what we stand for and what we believe.
Matthew 10:34 "Think not that I am come to send peace on earth: I came not to send peace, but a sword." It is not popular to take a stand for separation but I can tell you that God's Word is full of warnings about it. Are you living a separated Christian life or do you live daily leaving people to wonder who you are serving and what you believe?