Epiphanius of Salamis

Epiphanius wrote the Panarion — 'medicine chest' — a catalogue of eighty heresies with descriptions and refutations. It's the early church's heresiology reference book. Read it and you find out what dozens of forgotten Gnostic and Jewish-Christian groups actually believed, because Epiphanius quoted them at length to refute them. Without him, our knowledge of second and third century Christian diversity collapses. He was also a hardliner — anti-Origen, anti-icon, anti-anyone-who-disagreed-with-him — and his judgement is often unfair. But as a source he's irreplaceable. Half of what we know about lost heretical movements we know because Epiphanius hated them and kept the receipts.