He describes it himself in his Confessions, which is a great read for any Christian. I think Bishop Ambrose was highly influential on him, and he had spent over a decade of his twenties+ searching different philosophies and religions for the truth. Actually reading Paul’s letters really moved him, too. He found the truths he found elsewhere incorporated in Paul’s writings and the Christian faith as a cohesive whole, and it resolved the philosophical quandaries he struggled with.