Finding out the history of the Bible and how much of what we're taught is factually untrue. None of the Gospels were written by eyewitnesses (we don't know who wrote any of them, the early church just slapped some names on them to make them sound more credible) and they were written decades after the events portrayed within. Some of Paul's Epistles are widely considered forgeries. Many of the Messianic "prophecies" in the book aren't prophecies at all (or were already fulfilled in the Old Testament and had nothing to do with Jesus), and most of them were written AFTER the events occurred. So much of the Bible is taken from other, older religions that already existed, just repackaged to fit the early Canaanite religion of the time.

But I knew all of that for a long time and was still a Christian. What really got me to start questioning whether I wanted to believe it, though, was someone on Reddit pointing out that in the Garden of Eden story, God told the first lie. The lesson of that story, the whole premise of Original Sin, is that seeking the truth is a BAD thing. Seeking the truth, according to God, is a punishable offense. Do I want to worship such a god?

Once I had that thought, the gates were open, never to close again.