I'm guessing this is an already-existing marriage, rather than a potential one. In the latter case, I would simply say 'Run in the other direction.'
The Scripture is clear: to the Christian wife, respect your unbelieving husband. To the Christian husband, love your unbelieving wife. If the unbeliever is willing to stay, the marriage stays. If the unbeliever leaves, the believer is free.
A local Christian counselor who has seen many couples in this situation says it is one of the most difficult challenges a marriage can undergo. Pray much, and let the gospel be evident in your actions and attitude at least as consistently as in your words.