I was born into a Christian home, and sent to a fundamentalist Christian church. I never even thought to question Christianity until I was about 14, I just accepted every Christian premise I heard without question. When I was about 14, a deacon at the Southern Baptist church I was a member of, filled in for the youth pastor who was out sick. He asserted that the world was only 6000 years old, and that dinosaur bones were planted in the ground by Satan to trick scientists into leading people away from Yahweh. The first thing that shook me up, was the fact that he was not some random guy who just attended church sometimes. He was a deacon and a pretty high ranking guy in the church. I quickly turned the same rational eye that I had on his arguments onto my own beliefs and realized that they were just as silly. I told the regular youth pastor that when he got back. He challenged me to read the Bible cover-to-cover. I read it over the course of about ten days.
One day, I was taking a shower after returning home from a church service. While I was cleaning myself, I formed sort of a mental Venn Diagram of everything I had read in the Bible. I realized that almost everything in the Bible fit into a set of things which were horrible, or a set of things which were nonsense, and that most of it fit into the intersection of those sets. When I got out of the shower, I came to the conclusion that Yahweh did not exist, the Bible was a useless book of nonsense and that every Christian premise or argument I had ever heard was false.
A little later, I encountered Kent Hovind videos and a now famous Youtuber named VenomFangX. This was back when he only had about a thousand subscribers, and would respond to people’s private messages to him. I pointed out some of the flaws in one of his videos to him, and he replied with something to the effect of “Nu-uh, magic”. I refuted that argument, and he blocked me. While reading through the comments on another one of his videos (or maybe a Kent Hovind video), I found a link to the Skeptics Annotated Bible. The SAB (Wonderful resource when debating Christians) helped to solidify my conclusions about Christianity.
My explicit rejection of all other religions came after reading The Iliad. I enjoyed the book, and realized that it was pretty much the same thing as the Bible. A chain of events, some probably based in history, others completely fabricated with arbitrary deities from the region it was written inserted throughout. Reading it caused me to view religion on a more global scale and realize that every religion I knew of was pretty much isomorphic to Christianity, which I had already established a solid basis for rejecting.
I don’t think I have to justify my rejection of the sort of “New age” spirituality stuff. Anybody who has been exposed to it, and not immediately seen it as meritless garbage is a mental deficient.
tl;dr: Some Christian beliefs crazier than the Christian beliefs I held, got me to look at religion rationally.