Let's just be clear what we're talking about. This is a question asked by Christians. The issue at stake is can they "go out with" someone who isn't a Christian.
By "going out" we're talking about a temporary relationship. Going out is not a permanent state of human relationship. A couple either marries or breaks up. Its not a state of relationship we can find in the Bible and it is true that there is no direct command in scripture about this....
What we need to do then is see what markers we are given in this area.
Marriage is God's permanant state of relationship for a man and woman to enter into. It is not for everyone but it is a provision from God for some. Genesis 2 tells us that in God's very good creation there was something that was not good. The
Man was alone. Not lonely but in need of a helper. The man needed someone suitable to help him - literally a like-opposite. Animals are not able to do this, and so God creates Woman, from Man.
The woman is the helper that the man needs. She is the one who will help him to serve God - to complete the mandate given by God. This is not some subservient second-class slavery - but a mutual partnership in which God can be served.
What bearing does this have for us?
1. Firstly it is simply obvious that a going out relationship that works will end up in marriage. [edit: by works, I'm meaning "is worth staying in" - evidently not every "going out" relationship results in that, and it would be dangerous if going out became synonymous for "engaged", or even "married"]
Many people seem to enter into "going out" without considering anything beyond tomorrow. I would suggest that it would be careless to start going out with someone that you could not contemplate marrying. What would be the point? Friendship can be developed without going out and it is not necessary to test the waters of compatibility. God has said woman is man's helper, man and woman are made for each other.
2. Secondly then, since the purpose of marriage is for a man and a woman to help each other serve God both parties must be committed to this. It is evident that someone who is not a Christian is not desiring to serve God. If they were, they would become a Christian. So on this count alone a non-Christian and a Christian do not make a "compatible" partnership for marriage. They will pull in opposite directions.
At this point many object!
There are two common objections. One is subjective, the other pragmatic.
1. The subjective response is for someone to say: but God told me that it was ok. Well, actually - NO. God does not sanction sin. You need to know your heart, and liberated from sin as it is, it is still very capable of deceiving you, especially in matters of the heart. Don't kid yourself on this. God takes your relationships and your sin seriously.
2. The pragmatic response is that they've seen people do it and the non-Christian has become a Christian. This is self-evidently something that does happen. However just because God graciously brings good out of a bad situation doesn't make it right. One of my best friends did this and it worked out for him. My Mum did this - and I now have a Christian father, and I exist because of it. Doesn't make it right. Christians do not live by pragmatic theology, but by Biblical theology. What is right to do is what is Biblical, not what works.
(In the same way, not all non-Christian:Christian relationships end in conversion. But let me not use that pragmatism in my argument. Equally, it is not true to say that if someone goes out with a non-Christian they will lose their faith.)
The underlying issues here are about God's purpose for Christian marriage and the Lordship of Christ over the life of a Christian:
1. God's purpose for marriage is to be a relationship that reflects that of Christ and the church - a faithful relationship. (Quite different from the rather unique testimony of Hosea and Gomer, the faithful one and the harlot). Such testimony can only come from a marriage where both parties seek to live under Christ.
2. God's purpose for Christian life is that it be lived under his authority. Jesus Christ is Lord is the testimony of every Christian, and that includes every aspect of life.
Relationships are an area over which Christ should be Lord. It is not viable to say that Christ can have the rest, but I will do what I want when it comes to relationships.
Finally, let me grant myself one pragmatic point. Marriage is wonderful but also difficult. That my marriage consists of two sinners living under the same roof, two forgiven sinners, is one of the cornerstones that holds it together. I cannot imagine the consequences where only one party understands this. The assurance that comes from both my wife and I knowing the grace of God to us binds us together and drives us forward. There is always forgiveness, grace is enough.