Augustine of Hippo (354-430), an early Christian bishop and theologian.
In the engine's read, he supplied the ideas that let the Church survive Rome's collapse and justify using force against those it deemed heretics.
During a dispute with a rival Christian group (the Donatists), he built the legal and intellectual case for the state punishing heretics. His book The City of God (426) framed the Church as an eternal 'City of God' that would outlast temporal Rome.
That framing gave the Church a way to keep going as an institution even as the empire around it fell apart.