2 Corinthians 6:14 - Do not be yoked together with unbelievers. For what do righteousness and wickedness have in common? Or what fellowship can light have with darkness?
Paul wrote this passage to believers in Corinth, who were starting to listen to false teachers warning them not to attach themselves to a teacher who was not of Christ because that would be like trying to find common ground between righteousness and wickedness, or to make light and darkness friends. The big idea here seems to be that believers should not be lead or taught spiritually by unbelievers because believers and unbelievers are polar opposites. Thus, believers should not connect themselves to someone who will lead them astray spiritually.
We are all teachers in life. The people who are closest to you tend to teach you the most. Thus, those whom you date will impact or at least describe your spiritual walk, whether for good or for ill. If your boyfriend or girlfriend is not a believer, you are trying to make friends between light and dark. It doesn't make sense, you are the temple of God and they are of this world. They may be fun for a little while and make you feel special but ultimately, you come from different worlds. As believers we are to make it our goal to please God. For the unbeliever, that goal will not even be on their radar screen. That problem will bring significant strife with it in the future, if you would decide to get married in spite of the scriptural mandate (I Cor. 7:39) not to marry an unbeliever. You will find that you and your unbelieving spouse do not have the most important things in common, which will produce significant friction in your marriage. Most importantly applying the above passage, dating a non-believer is in opposition to the word of God.
Since the goal of dating should be marriage, to ultimately find someone that you would like to marry, the most important thing is that your future spouse be a follower of Jesus. If that's not the case then there is no reason to waste your time and the time of this other individual being exclusive with him or her. Keep yourself open, get to know lots of people until you find someone who you fits the criteria of godliness and whom you think you might like to spend your life with.
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