When I began my walk with the Lord I was still in a relationship with a non-christian. We had been together for over 2 years, me becoming a Christian was an obstacle in the relationship for him, and the relationship became an obstacle for me in my new life since he had asked me to 'wait' for him to 'catch up'. But God always provides wether it is spiritually or physically, and He did so spiritually when during that time at the youth group I went to on saturdays at my church, our youth leader began a series on relationships. I realized why it was so wrong for a Christian to date a non-Christian, and I learned what God had to say about it in His word. After much prayer the relationship ended, and it was no longer an obstacle in my walk and finally I was able to continue growing in my walk with the Lord. The following excerpt+link makes an excellent point explaining the reasons why christians and non-christians cannot join together in a relationship.
Dating: When Words and Choices Clash
by Dan Phillips
"Coarse but pointed joke-that-you've-all-heard-anyway alert in 3... 2... 1....So this iconic guy's talking with an iconic girl, and asks whether she'd be willing to engage in carnal acts for $1 million.And now, a moment's pause while Pyro readers wonder where this could possibly be going. Play the overture from Handel's Messiah in your head. Daaaa.... da-daaaa....
She considers, shrugs, says, "For a million dollars? I guess."
"Would you do it for $5?" he follows up.
She is shocked, and deeply offended. "What do you think I am?" she rages.
His classic retort: "Oh, we've already established what you are. Now we're haggling about price."
Now, why is that joke funny to most folks? It's funny because of the surprising clash. In the punch-line, the woman is deeply offended, because she wants to say that her virtue is precious, and he's cheapening it with his $5 suggestion. But the man counters that he already knows her virtue not to be beyond price to her. She can be bought. What she says now is one thing; what she has already said, another.
So you, unmarried Christian reader, start dating an unbeliever. What are you saying?"