Athanasius of Alexandria

Athanasius spent forty-five years as bishop of Alexandria and was exiled five times for refusing to compromise on the divinity of Christ. When the empire wanted unity-at-the-cost-of-doctrine, he chose doctrine and lost everything. He's why the creed says Jesus is 'one in being with the Father' rather than something fudgier. His short book On the Incarnation is the simplest, sharpest answer ever written to 'why did God become man,' and the reason C.S. Lewis insisted every modern Christian read at least one old book a year.