Thomas Aquinas (1225–1274) was a dominican priest and perhaps the most well-known and the most influential religious philosopher of Christianity. He was one of the scholasts, the religious philosophers of 12th-14th century Europe who worked on resolving the conflict of faith and reason by attempting to merge the Catholic religious doctrine with Greek philosophy, both inspired and appalled by the earlier work of Averroes who joined Greek philosophy with Islam in ways that were officially condemned by Roman Catholic Church. While Franciscan monks, led by Bonaventure, attempted the fusion of Christianity with Plato’s theory of Forms, the Dominicans followed Averroes in choosing Aristotle.
Bringing Aristotelian empiricism into religious philosophy, Thomas Aquinas stated that humans have the capacity to know truth on their own, without divine revelation, through observation and reason, which he called “natural revelation”. In his view, even the concept of God was discerned by reason alone, while the supernatural revelations added only the details about the divine, such as the nature of the Trinity or the purpose of Jesus Christ. In the teaching of Thomas Aquinas, faith and reason are one; study of nature is the study of God, and study of the supernatural revelations of the past is science.
The most commonly cited creation of Thomas Aquinas is the Quinque viae, the five proofs of God, which, as he believed, prove the necessity of a supreme being by reason alone. These five proofs were: unmovable mover (ex motu), first cause (ex causa), contingency argument (ex contingentia), greatest degree of perfection (ex gradu), and argument from design (ex fine). These arguments became an inseparable part of Christian theology and are reused again and again by the religious apologists today, although many philosophers, from Kant and Hume to Dawkins, were able to prove them wrong with ease.
Despite the mistaken views on the rationality of God, the philosophy of Thomas Aquinas was a boon to the Western Civilization; once the Catholic Church was convinced that study of Nature was study of God, the natural philosophers were no longer seen as heretics, and were now free to begin the scientific revolution.