"Do not be unequally yoked with unbelievers. For what partnership has righteousness with lawlessness? Or what fellowship has light with darkness?" - 2 Corinthians 6:14
There is no moral code attached to such a thing, it is not a sin to choose a spouse who does not believe as you do. But Scripture makes it clear that that is a hard, hard road to go down. For one thing, without a common moral foundation, there is no way to guarantee that your marriage vows mean to the same thing to one another. If each of you doesn't hold Christ in higher regard than each other, then how can you guarantee faithfulness?
God is a loving and compassionate God who is capable of softening any heart - but to take that sort of task on yourself and marry someone hoping that they will convert is asking for a lot of heartache. The marriage relationship is a partnership, a constant and mutual servitude, and the covenant itself as an expression of God's love for humans. To bind yourself to someone who doesn't understand that, or doesn't place the same value on it, is a very painful risk.