Question:I am a Christian dating an unbeliever. Is it a sin?
God's Word does not necessarily come out and say "Thou shalt not..." but it does give us some great advice.
2 Corinthians 6:14 says this, "Don't be yoked together with unbelievers. For what do righteousness and wickedness have in common? Or what fellowship does light have with darkness?" The main point being made is that we need to carefully examine our relationships with non-believers. We should not be "yoked" or "joined together" with those who are not believers.
The application is in the form of a question: "If Christ is the most important thing in your life, why be involved with someone who has no regard for the very thing that is so important to you?" If you can't share your love and commitment to Christ with this person, which is at the deepest part of you... what do you really have to share? Unfortunately, in dating a non-believer, we lack the intimacy that makes a relationship work. This is not to say that you cannot be friends, but once you introduce "dating" into the relational mix, you could be asking for trouble.
In my own personal experience, the times that I have dated non-Christians has only brought tough times, pain, and compromising of my faith and values. That relationship is so primary and so vital, that I believe that to date a non-Christian is very dangerous business, one that we would do well to leave behind.