When the LORD thy God shall bring thee into the land whither thou goest to possess it, and hath cast out many nations before thee, the Hittites, and the Girgashites, and the Amorites, and the Canaanites, and the Perizzites, and the Hivites, and the Jebusites, seven nations greater and mightier than thou; And when the LORD thy God shall deliver them before thee; thou shalt smite them, and utterly destroy them; thou shalt make no covenant with them, nor show mercy unto them: Neither shalt thou make marriages with them; thy daughter thou shalt not give unto his son, nor his daughter shalt thou take unto thy son. For they will turn away thy son from following me, that they may serve other gods: so will the anger of the LORD be kindled against you, and destroy thee suddenly. (Deuteronomy 7:1-4, emphasis mine)
In high school I once took a survey in which one of the questions was something along the lines of "Would you marry someone of a different religion?" At the time I was an atheist and determined that should I meet a Christian I think we would get along fine, this quite honestly was based upon the fact that the "Christians" I knew didn't really live their faith, they didn't seem any different to me than anyone else. They didn't talk about their faith, they didn't live differently, they just went to church on Sundays. And so I thought it would be fine for interfaith marriages to take place.
Years later I've realized that if one truly loves God, if one truly desires to live faithfully serving Him, an interfaith marriage probably isn't the best place to be. Marriage is tough, serving God is tough when surrounded by people who want you to just give a little...bend a little....compromise a little. Tough choices have to be made in marriages on their own, you have two lives coming together trying to learn how to become one, add opposing faiths into the mix and it gets even more difficult. We see a perfect example of this in Solomon.
But king Solomon loved many strange women, together with the daughter of Pharaoh, women of the Moabites, Ammonites, Edomites, Zidonians, and Hittites; Of the nations concerning which the LORD said unto the children of Israel, Ye shall not go in to them, neither shall they come in unto you: for surely they will turn away your heart after their gods: Solomon cleaved unto these in love. And he had seven hundred wives, princesses, and three hundred concubines: and his wives turned away his heart. For it came to pass, when Solomon was old, that his wives turned away his heart after other gods: and his heart was not perfect with the LORD his God, as was the heart of David his father. (1 Kings 11:1-4)
I believe this is the very reason Paul writes to the Corinthians, when we join hands with people who aren't serving God we can become corrupt. This goes beyond marriage though, it touches every aspect of our lives. Marriage, friendships, buisness dealings, everything. To join together means someone is going to have to compromise, and since our flesh loves the world it can be very easy to give into our old natures and let a little worldliness creep into our lives for the sake of getting along. But God tells us to come out, to be separate:
Thus saith the LORD of hosts; Ask now the priests concerning the law, saying, If one bear holy flesh in the skirt of his garment, and with his skirt do touch bread, or pottage, or wine, or oil, or any meat, shall it be holy? And the priests answered and said, No. Then said Haggai, If one that is unclean by a dead body touch any of these, shall it be unclean? And the priests answered and said, It shall be unclean. Then answered Haggai, and said, So is this people, and so is this nation before me, saith the LORD; and so is every work of their hands; and that which they offer there is unclean. (Haggai 2:11-14)
Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness? And what concord hath Christ with Belial? or what part hath he that believeth with an infidel? And what agreement hath the temple of God with idols? for ye are the temple of the living God; as God hath said, I will dwell in them, and walk in them; and I will be their God, and they shall be my people. Wherefore come out from among them, and be ye separate, saith the Lord, and touch not the unclean thing; and I will receive you. (2 Corinthians 6:14-17)
God does not tell us to be separate because He is trying to spoil our fun or make our lives difficult, He tells us to be separate because He loves us! He knows the company we keep will effect the way we live and the way we love Him. He calls us to separate because He knows how easily we can be drawn away, He calls us to live a life of separation because He loves us and wants us to follow Him. It is in following Him, in loving Him "with all thy heart, and with all thy soul, and with all thy mind," (Matthew 22:37) that we are safe. The world does not love us, not with a true and faithful love that would sacrifice itself in order to save us. But God does. He hasn't chosen us because of anything we have done, He hasn't loved us because we are strong. Indeed He loves the lowly, the poor of spirit, the humble, the few of number, the widow, the fatherless child...He loves us because in these He is glorified. When we walk knowing the strength we have is not our own, when we rejoice in the cross, when we realize there is nothing any of us can do to earn His love and realize He loves us anyway, He is glorified.
Would He be glorified in a people who don't need Him? This is why He chose the Israelites, poor slaves, to be His chosen people. This is why (in part) they spent 40 years in the desert. He is glorified in our realizing how much we need Him. And the world denies this, they deny God, they deny His power, they deny the fact that they need Him. He calls us to be separate because the world would have us believe that we don't need Him either. They would tell us the good in our lives is because we worked for it, because we earned it. They would have us think that we need nothing but ourselves, that we must look to ourselves for salvation, that if we all just work together we can make the world perfect and full of peace...all without the intervention of God. Ah, but we know this isn't true. All of scripture shows how desperately we need to lean upon God and allow Him to lead.
This is why He calls us to be separate, because if we surround ourselves with people that deny the very power of God we probably will be drawn to think we do things in our own strength and if we begin to think and act like the world this doesn't speak well of the transforming power of the cross. If we live just like the world, how is He going to be glorified in us? And so if we really want lo follow God, to serve God, to glorify God, we must be separate.
For thou art a holy people unto the LORD thy God: the LORD thy God hath chosen thee to be a special people unto himself, above all people that are upon the face of the earth. The LORD did not set his love upon you, nor choose you, because ye were more in number than any people; for ye were the fewest of all people: But because the LORD loved you, and because he would keep the oath which he had sworn unto your fathers, hath the LORD brought you out with a mighty hand, and redeemed you out of the house of bondmen, from the hand of Pharaoh king of Egypt. Know therefore that the LORD thy God, he is God, the faithful God, which keepeth covenant and mercy with them that love him and keep his commandments to a thousand generations; (Deuteronomy 7:6-9)