C.S. Lewis, specifically The Screwtape Letters. I had been raised very conservative Catholic, and this book was my introduction to moral philosophy, as odd as that may sound given the overtly religious nature of it. The idea that morality has nuance, that an action can be wrong and still not damning, or 'virtuous' and still evil, was a new idea to me in my early teens.
While it would take me several more years to really start learning more definite philosophical concepts, that book was the first one that actually challenged me to ask myself why I believed the things I did, and made the case that blind, unchallenged faith was not faith at all. I started paying more attention to the things I had previously accepted at face value, and that examination would lead to me leaving the church and Christianity entirely later on. I still have faith of a sort, but it is more a faith in humanity and an undying and unifying spirit of community than a religion.
Now I have read quite a bit more in terms of philosophy, though not as much as I would like. All thanks to one book about demons trying to send a man to hell.
Saint Francis of Assisi did this. He renounced his family name, inheritance, and (according to legend) the clothes on his back when his dad took issue with him giving alms. He spent the rest of his life wandering with a small group of other penitents and providing comfort to the sick, especially those with leprosy.
https://en.wikipedia.org/wiki/Francis_of_Assisi?wprov=sfla1