Looking back, I had little questions all along, but pushed them aside, figuring that smarter people than me had sorted it out already. That willingness to go with the flow started going away when my first marriage to a Good Christian Girl(tm) started to unravel, and I was presented with my first heavy load of cognitive dissonance that arose from how real life goes vs how it's "supposed to" go for a devout believer. Things only got worse, and at some point, the pain of not changing my views was greater than the pain of changing them -- so I did. That happened gradually over the period from my late 20s to my late 30s, and I did not fully self-identify as an atheist until my late 40s.

I was never a believer, I was lucky enough to not have been raised in a religious household.
So I never had the "eureka" moment when I figured it out out so to speak.

The first inklings of non-belief were caused by Sci-fi. This includes both books and Star Trek TOS. I remember one year one of our local TV stations showed Star Trek after school - and I remember watching it on a black and white TV (this was in the 70s and we did not have enough money to buy a color TV). After watching them again and again over the years I was struck by episodes like “Return of the Archons”, “The Apple , and “who mourns for Adonis?” andd how gods/religion were portrayed. #startrek

My grandmother was Baptist’s my dad was Catholic and the converted to Judaism so once I asked “ why are there so many different religions for only god? “ and received some very invalid and inconsistent answers I knew that there was some bullshit behind this.