You simply need to talk about this with her to find out if she's cool with it. Also, if your relationship progressed, would you be cool with your kids going to church? Keep in mind that Catholicism, especially these days, isn't like being a witness. It's a lot easier for religiously divided families if the religion isn't a cult like the one you have known. Also, you're agnostic, not atheist, which makes the religious divide a little narrower. Would you go to church with her and support her? You wouldn't have to do the faking thing, you could just explain you're an agnostic to people and they would be ok with that. A lot of it is your attitude. If you can respect her beliefs and not belittle them, and if she respects that you don't think her religion has all the answers, this could work.