I knew Evangelival Christianity wasn't for me around the age of 14, but I seemed to move through the spectrum of Christianity, general theism, and spirituality before I finally decided I was an atheist about one year ago. It was relatively easy for me to drop Christianity, but I towed the theist agnostic line for several years before reaching a tipping point.

For me it's been less that 3 mouths. However, I wish i was closer to 3 years or better.

Sorry ...this story grew longer than I had anticipated.

Well as a result of centuries of inbreeding I am short and I was even shorter during my childhood. So my growing up was slow and rather unsuccessful. However, when I was 10 I had reached my grandmother's, my mother's adoptive mother, level and could talk to her eye-2-eye. I think it was on a Saturday when I realised.
I planted myself in front of her and said, obviously very convincingly, that I was big, big as in adult. She just nodded and said in a rather hesitant voice: "So?"
Me: "Well, from now on I don't have to accompany you on your dominical procession to the church, grantherfather's tomb and Frau Kurt."
Her answer was as prompt as it was short: "Fine!"
The next morning I was for the first time all alone in a big house, a treasure cove of mysteries.
BTW this newly acquired status also allowed me to move into another bedroom. For years I had to sleep in my granfather's bed in which he had died.
So it was the first time in my life, excluding baby years about which I remember precious little, that I had slept in a room all alone.
When I woke up I just heard how granny shut the door behind her. I rushed to the window to check whether she was really leaving or just pretending and waiting behind the door. She wasn't. I saw her already crossing the road. At that moment I really loved that little plump oversized soft toy.
Just then I felt really emotionally attached and I realised that I had never appreciated an aerial view of her.
After this first surprising moment of discovering my feelings for that strange being I thought it was time to get into action. No bothering too much about hygiene I just slipped into my clothes and into the lounge room which must have been my grandfather's officeMy head felt like a carousel, some thoughts seemed to drift away by centrifugal force only to bounce off my meninges.
My opened the door large bookcase/cupboard. On the lower shelf was a pile of magazine pages very neatly piled up. Almost instantly I realised that this was the result of granny's meticulous execution of her censorship task. I don't even wether I had ever realised that the magazines she had always passed on to my mother had been subject to rigorous censorship. I only remember my father correcting her crossword mistakes.
Here I was standing in amazement. I imagine the people seeing the Berlin Wall come down couldn't have been more amazed. I was looking at a piles hundred or even thousands of photos of naked women whose shapes had nothing in common nor with my granny's nor with my mother's.
Somewhat timidly I took a few. I realised they were in chronological order. So thought it was safer to extract a few from within the pile. I felt a little shaky as I lifted a pile of several hundred pages and carefully set it down. Then I grabbed several dozen and replaced the top pile. Shut and locked the door. I swiftly carried the pile upstairs into my room and started to look for a good hiding place for this precious treasure.
If it had been needed this little episode convinced me that there was definitely no god. Rather than being punished for not going to church I received the heavenly reward of tons of picture depicting divine creatures. Hallelujah!!! Praised be my granny!!!

Since I was in my teens, and started reading the bible for myself instead of letting others, including the Catholic Church, tell me what it says. The more I read, the more my already-developing bullshit detector became powerful

Basically this was my deal:

And that was that.

I was never in a religion in the first place. My experiences with church were limited to Deaths, Marriages and some weird shit in grade school where we basically went to bible school every Wednesday (don't ask, it's Indiana bullshit).

Since l was 13 or 14

It has almost been a year and a half coming out as an atheist.

I was around 6...during an evangelical sermon on Noah, I just knew the Bible/etc was the same as Santa. What amazed me as a child—still does—is that grown-ass adults blindly bought it.

I was...I don't know...ten or eleven. My parents were non-religious (dad was an apathy atheist who theorized all religion as a means of social control, my mom was a lapsed catholic) but I was completely surrounded by people who had for years been claiming to belong to a religious philosophy of tolerance and love and compassion but who ridiculed, assaulted, and humiliated me at every turn because I didn't go to church and don't accept every word in the King James Bible as 100% literal accurate infallible fact. Every discussion of evolution and natural selection (which seemed upon examination of the information even then to be pretty a solid way for species to survive and thrive and confused me that other people couldn't separate evolution as a natural phenomenon with the very specific hominid evolutionary chain which led to humanity and offended their 'I was made by god to look like him[isn't that vanity and arrogance and hubris?]' Philosophy) turned into a 20 to 1 fight with me sitting all by my lonesome being told I was going to hell forever and they didn't come from no monkey and I'm the stupid crazy one. And any attempts to point out flaws and inconsistencies in the narrative are met with equal anger and aggression.

It's pretty easy to avoid a place you feel so overwhelmingly unwelcome, and that rational critical thought is so fervently unacceptable.

Your story parallels mine. My break was over 60 years ago.