My girlfriend's family wants me to be more formal.

@noah18 I'm not sure this is about being "more formal" vs. trying to convert you and/or make you more religious than you are already.

What is it that you object to? Going to church? Wearing certain clothes? Personally I'd compromise, especially depending on the nature of your relationship with them, what their church is like, etc. But if their chuch is less formal and people show up in like shorts and flip flops (I don't know, I've never been) then I'd say you have good standing to decline.

It's up to you. Personally I'd take this as a warning sign they're trying to change you to be more "presentable" in their POV. If you do, great. If you don't, then they might "encourage" their daughter to find someone more "suitable."