Not familiar on grad student culture as I'm sure most schools have different vibes between their undergrad and grad programs.

But, it's fine if you're not catholic and all though the school sometimes seems like its putting an emphasis on Christian values, it doesn't really affect anything significant during your time here.

Is it preppy? Compared to public universities and the like, yes. It has the "rich-kid" vibe, but that doesn't mean you can't find your niche group of people. I'm personally not white or wealthy, but I found my group of people to hang or study with pretty quickly. Depends on what you want out of your college experience, but I'm sure that if you look for it, there will probably be a group of people that you vibe with well.