The short answer: Greek philosophy, namely the Stoics.
I was raised as a fundamentalist Pentecostal, but abandoned its view of god, and faith altogether, as I started my twenties. I was still interested in religion and began studying them, trying to understand their strengths and how they operated. My notion of God was very old testament; I was attracted by His glory and power, yet repulsed by the cruelty of the Pentecostal creed and of the god it portrayed. In university I encountered the philosophy of the Stoics, and – after employing Stoic practice to great effect in my own life – began to study it in earnest. I began to share the Stoic appreciation of god, not as separate from the Cosmos but his logos constituting it. At the same time I was reading about Gandhi and Martin Luther King Jr, considering the radical love at the heart of their lives, and of Jesus’ though I avoided him considering my tortured history with Christianity. Although I did not believe in anything at the time, the old imaginary-friend-in-the-sky-who-sometimes-smites-you Pentecostal version of god had fallen away to a more noble understanding, one that restored my reverence for the majesty of YHWH.
Then one day, brooding over the betrayal of a friend, waxing self-righteously angry about his behavior toward me, I realized for all my moralizing I was just as sinful as he was; I thought the same thoughts, I just did not give them voice. I dwelled on them, but I was too vain about my own morality to ever give anyone reason to doubt it. This was the first time I’d ever felt like a capital-S Sinner, and what I wanted more than anything else was redemption. At that moment the full weight of divinity hit me, and I felt as though I had been walking for many months on a trail at the base of a mountain without knowing it, and now the fog had cleared so the point that I could fee lit. Eventually it sank in that my yearning for redemption was a Christian vision, so I put my fear aside and began to study Christianity anew. A friend of mine was converting to Catholicism at the time, so I joined him in a Catholic PalTalk room and began attending liturgical services for the first time. I’m not Catholic, but a kind of high Anglican. I listen to Catholic podcasts and read both catholic fiction and nonfiction, though, so…who knows where the road might yet go. My experiences have convinced me that the light of God shines on all men, as surely as the sun illumines the whole globe. I could not be a Christian but for the Stoics, Gandhi, and Khalil Gibran.