Augustine of Hippo

He was a Manichaean Hearer for nine years, from nineteen to twenty-eight, and left partly because the movement's great teacher Faustus could not answer his questions about astronomy. The rest of his life is in large part an argument with the position he used to hold.

The best inside witness in the corpus — hostile, but hostile from the inside. And his eventual answer to evil (a privation, a lack, not a substance) is a direct rejection of two-principle dualism, and the road Western Christianity took instead.