What should I do if my girl friend wants me to go to church but I don't believe in God?

I was raised catholic but really stopped going to church and believing in God a long time ago. My gf now wants to me to start going to church with her and I don’t know how to tell her this. What would you do? (ps I know the best thing I can do is talk to her about it rather than ask a room full of strangers, if that was your comment I just saved you from typing it. Thanks! tom)