Cyril of Alexandria

Cyril is the most brilliant and the most ruthless. His Christology — one person, two natures, a real unity in Christ — won at the Council of Ephesus in 431 and shaped Chalcedon twenty years later. Every later orthodox account of who Jesus is starts from Cyril. He's also the bishop in whose Alexandria the philosopher Hypatia was murdered by a Christian mob in 415, and he ran Nestorius into the ground with political tactics that were ugly even by fifth-century standards. The church canonised him anyway because his theology held. Read his On the Unity of Christ — it's the cleanest statement of how Christ can be one without being a blend.