How do I tell this guy no?

I like this guy. Being impulsive.....I gave him a note telling him that I like him. He's coming back from his Italy trip (with the school) on Tuesday, which means he will be at Tennis practice. (By now he received the note.) uming he likes me and asks me out, what should I do? He doesn't believe in God and I do, so I can't be with him. When he asks me why I said no if I like him more than a friend, then what should I say? If I tell him it's because he doesn't believe in God....he might think it's stupid. I don't feel like going around high school feeling stupid.....It's already bad enough that I'm a freshman. :/