Thomas Aquinas

In the 13th century, Aristotle's complete works re-entered Latin Europe via Arabic translations. Some theologians were terrified; the works seemed to threaten Christian doctrine. Aquinas saw an opportunity. He devoted his enormous Summa Theologiae to showing that Aristotelian philosophy and Christian revelation, properly understood, do not conflict – they complete each other.

He developed the Five Ways (five arguments for God's existence) which remain the most-cited natural-theological arguments. He distinguished what reason can show (that God exists, that the soul is immortal) from what reason can only receive on revelation (the Trinity, the Incarnation). He turned Aristotle's virtue ethics into a Christian framework that still grounds Catholic moral theology.

A late mystical experience – "all that I have written seems like straw to me" – caused him to stop writing three months before his death. The Summa was left unfinished, which somehow makes it the more monumental.