“THE Church was not a Manichean movement if only because it was not a movement at all. It was not even merely an ascetical movement, because it was not a movement at all. It would be nearer the truth to call it the tamer of asceticism than the mere leader or loosener of it. It was a thing having its own theory of asceticism, its own type of asceticism, but most conspicuous at the moment as the moderator of other theories and types. This is the only sense that can be made, for instance, of the story of St. Augustine. As long as he was a mere man of the world, a mere man drifting with his time, he actually was a Manichean. It really was quite modern and fashionable to be a Manichean. But when he became a Catholic, the people he instantly turned on and rent in pieces were the Manicheans. The Catholic way of putting it is that he left off being a pessimist to become an ascetic. But as the pessimists interpreted asceticism, it ought to be said that he left off being an ascetic to become a saint. The war upon life, the denial of nature, were exactly the things he had already found in the heathen world outside the Church, and had to renounce when he entered the Church. The very fact that St. Augustine remains a somewhat sterner or sadder figure than St. Francis or St. Teresa only accentuates the dilemma. Face to face with the gravest or even grimmest of Catholics, we can still ask, 'Why did Catholicism make war on Manichees, if Catholicism was Manichean?'"
~G.K. Chesterton: The Everlasting Man.