They gasped & crossed themselves.

Who does that besides you? And Why?
Simple enough if the occassion warrants it to just say,, "I'm not religious" & move on.
Do you think there should be cake or something?
Are you really an atheist or just someone who likes to stir up trouble? Totally pointless, I assure you!

It wasn't their business

I'm not an atheist, I'm agnostic. Why are you assuming everyone who reads your words is atheist?
I didn't "come out" as agnostic, this isn't like sexual orientation. I've been always been agnostic, though there did come a time I consciously realized it.
But I didn't go around announcing it. And why would atheists "come out?" Anyone who does so is being ridiculous. Are we supposed to hold a press conference?

I really didn't really have anyone to come out to. If someone ask me, I will tell them an atheist.

Well, for me it was kind of a double whammy. I came out as atheist at the same time I came out as gay. My intent was to come out gay while taking away any religious arguments people might have. It mostly worked. There are some who said I left god to justify my "lifestyle". Even though I had stopped going to church years earlier, that is how they try to make sense of it in their own minde.

I have never encountered anyone that was surprised.

No-one has ever taken me anywhere just because they've found out I'm an Atheist....LOL.
But, I can say quite a few Faithnuts have told me where to go.

Never really did, in the UK atheist/agnostic is pretty much the default.

Some family accepted it…….friends and colleagues as well.

Nobody has ever given a fuck about me so it was mostly uneventful.

My two daughters accepted it but later went more towards religion again. This was either because of Trump or the pandemic. I actually came out atheist around 2012. My religious friend from age 12 knows my background and thinks I am "just angry with god." My mother had already passed but my now deceased stepfather told me I "was going to split hell wide open." None of this stopped all of us from talking but they have their own made up "truth" about my de-conversion.

My best friend ghosted me. How he didn't catch on earlier is beyond me. He'll get over it or he won't.

Some were fine with it, others were worried for me or critical of me for it. I really didn't care if they were negative about it as long as they didn't hassle me or try to make me change my mind.