Sorry this is wordy. Like I said, I like these topics.
I was raised and K-12 educated Catholic. I don't know that I ever really believed...it went from something that a kid just parrots ("I believe in Jesus") to something that I started to really question in high school ("why am I saying this if it doesn't make any sense to me?"). I informally decided I could no longer call myself a Catholic sometime during college. I don't really label myself as an atheist, and I say agnostic if anyone asks, but in reality my beliefs - or lack thereof - fit pretty well with most people who call themselves atheists.
My fiancée knows; she considers herself Catholic but she goes to church on Christmas only. She doesn't care. My immediate family knows. My mother has taken it pretty poorly ("are you a devil worshiper?") but she's gotten better over time. I think for a year-or-two there, she was embarrassed that her kids are the "weird ones", because everyone in her friend group is Catholic, or at least pretends to be. It was like my lack of religion made her look like a bad parent. My brother is very vocally atheistic, whereas I don't really talk about it, so it's taken the brunt off of me a little bit. A handful of my closest friends know.
I got engaged about 5 months ago, the topic immediately came up about getting married in a church, and I "pulled off the bandaid" and revealed to my fiancée's immediate family my lack of religion. Her mother is not taking it well. I've heard the rumblings that she called all of her sisters and her other 3 daughters and basically bared her soul about how sad she was that her oldest daughter won't be married in a church and is marrying someone who doesn't believe. She hasn't said a word of that to me, and I've never had a problem with her, but I can tell she's half-stunned, half-upset about it. Like RHE says, I think they're probably dreading the prospect of baptism and such going forward, should we ever have kids.
I remember when my grandmother (dad's mom, RIP) was getting older and more frail, we knew she didn't have a lot of time left. She would spend her weekends at my parents' house, and my brother and I were over for dinner. My mom got on the topic of religion again, and made a comment to my grandmother about how my brother and I are going to hell because we don't believe. My grandmother, this tiny woman, super super devout Catholic, turned to the two of us and said something to the effect of "as long as you're both good people, live good lives, and you're happy, who am I to judge or tell you what to believe?" And that was that. She still loved us and it didn't change a damn thing. I'll never forget that. I wish everyone could have her mindset, whether they're a believer or not.