We are all works in progress and it's impossible to know how someone will respond to God's call in the future.
If I was a young Christian person today and I met someone who I was interested in but wasn't a Christian, I would invite them to church, or try to get them involved with a group of Christians, and see how they respond to the message. After a period of time I think it would be fairly obvious if they are interested in Christ or not. Just observing someone interact with other people will tell you a lot about their character. I also wouldn't communicate my interest, or try not to anyway.
If they come to Christ, great! If not, you at least witnessed to them and helped them to find the path.
Marrying someone who is out-right hostile to Christianity and in love with the world will cause a lot of problems later. Especially when children come into the picture. You will definitely be trying to pull the cart in two different directions.