It's a little late in the game, but the thing that pushed me over the edge was seeing my religious friends in their church.
I had been agnostic/ atheist leaning for a long time at that point just because my family didn't go to church and I was always able to follow my own logic and reason. My dad and grandparents were still religious though, so I never actively engaged in any public atheist behavior (not that there is much). Anyway I was that kid who called BS on Santa as soon as I learned the words to do so, but God and company were too deeply involved in larger society for me to fully let go for a while.
Upon entering tenth grade I made a new group of friends who were all church goers, Catholic in fact. Until this point all of my other friend groups were totally secular, so this combined with the environment I grew up in made me largely unaware of the activities of the church. This friend group talked about normal teenager stuff about 10% of the time, football 15%, and youth group, confirmation, etc. 75% of the time. The church was their entire life, they looked forward to being "confirmed" by an entity I soon came to be confident in the nonexistence of, they were sucked in and couldn't get out. There was no questioning the church or its teachings in their mind, often they'd come in talking about "guest speakers" who would tell some heartbreaking story about how abortions ruined their life or how a loved one is gay and might be going to hell.
It was like a massive cult, which recruited members from birth and trapped them inside. They never even had a good chance to question the world. It made me legitimately sad and just pushed me over the edge. I decided that the church was basically nothing more than a cult designed to further its own motives and that even if there was a God then the church is doing a pretty shitty job of serving it.
I came to the conclusion that there probably wasn't a God on my own, but realizing my contempt for the church severed any tied I had l had to religion and theology.
That said religion isn't all bad, it has some good teachings which everybody can benefit from.