The Gospel of Thomas. Before going down that rabbit hole I had no idea that Lucretius had laid out evolution in 50 BCE and would have never thought there was a sect and text claiming Jesus was talking about quantized matter, evolution, and pre-computer simulation theory in an agreement with the Epicurean rejection of intelligent design while rebutting their conclusion that there must not be an afterlife.
Not only did the study of the work itself lead to mind blowing realizations about history and philosophy, but the sheer absurdity of its existence has (for me) led to heavily complementing the physical arguments for simulation theory and pushed it over the edge from something just "interesting to entertain" to something I'm fairly confident in.
If you'd told me 6 years ago a 2,000 year old document would change my perspective of metaphysics and core beliefs, I'd have laughed you out of the room. And yet it today stands as by far the most interesting thing I've ever researched and likely the most influential to date.