Translators working from Arabic and Greek had just handed Latin Europe the complete Aristotle, and the world he described ran on its own causes, had no beginning, and on the reading then current in Paris left no individual soul to survive death. The best account anyone had of how nature actually works contradicted what Christians were required to believe. In 1210 a church council forbade the masters of Paris to read Aristotle's books on nature, in public or in private; the ban was renewed in 1215 and eased in 1231 only on condition that the books first be corrected — a correction nobody ever made. There seemed to be two options: ban the books, or find out that the faith was false.
The claim that made Thomas Aquinas is that this was a false choice. God authored nature and revelation both, so a genuine finding about the world cannot collide with a genuine article of faith, and reason must therefore be let run all the way to its own conclusions without being nudged. Grace does not cancel nature but completes it.
From where he stood, that position cost something to hold. His family were minor nobility of the Kingdom of Sicily; he was sent to Monte Cassino at five, and his uncle Sinibald held the abbacy that was meant to become his. At nineteen he joined the Dominicans instead — mendicants, men who begged. His brothers seized him on the road, and the family kept him almost a year in the castles at Monte San Giovanni and Roccasecca, sending a woman to his room to break his resolve. He left through a window. He had traded a lordly abbacy for a job in the new universities, arguing with strangers for a living.
He never took the safe route of quarantining Aristotle. He worked through the texts line by line, and fought hardest against the men who claimed to be Aristotle's truest readers. Against Siger of Brabant, the leading master of the Paris arts faculty, and the Averroists, masters who read Aristotle through his Muslim commentator Averroes, he wrote De unitate intellectus, against the proposition that one single mind does the thinking for all humanity — which would mean that no thought a person has is his own. And on the eternity of the world he conceded to the philosophers the very point his own side most wanted to win: that the universe had a beginning in time cannot be proved by argument; it is held on faith. He would not take a win he could not earn.
He began the Summa theologiae in 1265 and said it was for beginners. It runs to 614 questions and more than three thousand articles, and every one of them states the objections first, in the strongest form he can give them, before he answers. In December 1273 he stopped writing and told his secretary he could not continue. He died on 7 March 1274 at the Cistercian abbey of Fossanova, travelling to the council at Lyon, and other men finished the last part out of his earlier notes.
Three years to the day after that death, the Bishop of Paris condemned 219 propositions, some of them Thomas's. That same month the Archbishop of Canterbury — a Dominican, of his own order — prohibited thirty theses at Oxford, several of them touching his doctrine; whether he was the intended target is still argued. He was made a saint in 1323 and in 1567 a Doctor of the Church, an official teacher of Catholic doctrine, and in 1879 Pope Leo XIII called for his philosophy to be restored across Catholic teaching; the 1917 code of canon law turned that call into a requirement binding on every seminary. Being made compulsory did his reputation as a philosopher no favours at all. What revived him came from outside the church: since the middle of the last century, philosophers of religion, ethicists and theorists of action have taken him up as an opponent worth arguing with.