If you are a Christian, you need to consider other Christians for marriage, so that you will be equally "yoked". If you are living for Christ, and you marry someone who doesn't consider the Lord or Jesus to be important, you can be certain that it will either strain your marriage OR you will begin to fall away from God yourself.
[SUP]
2 Corinthians 6:14 [/SUP]Do not be unequally yoked with unbelievers. For what partnership has righteousness with lawlessness? Or what fellowship has light with darkness?
Since you would need to consider only other Christians for marriage, it is best not to date non-Christians, because there is a good chance you could come to love an unbeliever, and be tempted to set aside your spiritual values in order to be with him.