For me it was a long and layered happening.
Such as having a Christian education that taught me how the world is, and then I got outside of that extremely insular world and found that just wasn't the case. I also learned things in history that didn't match what the Church taught me, but it made more sense seeing things through that lens (though it came as a shock to seeing Christianity listed as a reason Rome fell, but as it was taught it made sense as I saw that wasn't an attack on Christianity), and I learned science really just has nothing to with any god or devil and that too wasn't the nasty "Satan's way of pulling people away from God" that I was taught it is, nor did any of it even make any effort or attempt at disproving god.
Another layer was being taught how to hate myself, deeply and profoundly, and I was utterly terrified that because I'm trans and having attractions towards men I was risking angering god and being cast into Hell. Which, in all reality, the preachers of such Churches are very much into threats of hellfire and brimstone, they regularly condemn others and even say other denominations are going to Hell, and I ended up having nightmares of going there (I can still remember the mental anguish and pain of fire burning my flesh).
There is also the layer of Jehovah's absence. I didn't have an easy or pleasant childhood, and nothing ever got easier or better. It was just another hardship or tragedy after another. Amd I didn't find any comfort, at all, in the story of Job because he was being challenged he had a comfortable life filled with happiness and family before his challenge. I didn't have anything yanked away. I was presented with anything new. My entire life just sucked, and despite my beliefs and prayers nothing got better. My parents divorced, my dad was alcoholic, my mom an abusive authoritarian, bullied and tormented, learning of betrayal at a young age, but despite all my pleas (including praying for death) I also got to go through my bus driver running over and killing my dog.
Reading the Bible for myself was also another layer. Not reading what others tell me read and how to read it, but just read it. I was looking for guidance, looking for that love and mercy, looking for inspiration and I found tyranny, great violence, harsh punishments, slavery and misogyny, war and genocide, and none of it was comforting me. It was only helping to push me away.
I remember, as I learning things about the world, I learned many of the OT stories actually came from other cultures. I learned a lot of it just could be true. But Christianity and Jesus was all I knew, and there at the end I even rejected the OT while still clinging to Jesus. But there was no reconciling that without the OT there is no purpose or reason for the existence of the NT, that ultimately without the OT there really can't even be an NT.
And after much inner turmoil and strife I let it all go and the healing could begin. And I seem to have been lucky because when I let it go I let it go and haven't suffered doubts or further fears since I did let it go. But it was such a tremendous burden off my shoulders that I don't think I could have second guessed myself if I wanted to. One second I was a Christian, the next I renounced it all and it was if a terrible curse were lifted from me and came to an end.
