This question split Europe in half. Can one believe and think at the same time? Or does faith demand that reason be switched off, and reason demand that faith be thrown out? Thomas Aquinas built bridges. Gregory Palamas said: God is higher than any bridge. Luther said: reason is the devil's whore. Descartes began with doubt — and arrived at God. Kant shut the door on the theological proofs — and opened a window for moral faith. The argument is not over.
Thomas Aquinas was the most ambitious bridge-builder in the history of thought. He took Aristotle — a pagan philosopher the Church was afraid of — and reconciled him with Christian theology. Reason and faith go to one and the same truth by different roads. Reason can prove that God exists. Faith discloses what reason would never have found on its own. This system stood for seven centuries — and still stands in the Catholic tradition.
Palamas defended the hesychasts — the monks of Athos who claimed: in prayer we see the uncreated light of God. His opponents said: God is unknowable, there is no light, this is psychology or worse. Palamas answered with a distinction: God is unknowable in His essence — but His energies are real and available. This is not irrationalism. It is another kind of knowledge. The Palamite controversy is an argument about what counts as knowledge.
Luther called reason "the devil's whore" — and was all the while one of the sharpest minds of his time. He meant something quite specific: the scholastic proofs of God's existence give nothing for salvation. Salvation is by faith alone, by grace alone, through a personal encounter with the text of Scripture. No intermediaries, no philosophical systems. It was a revolutionary gesture — and a very dangerous one.
Descartes began the "Meditations on First Philosophy" with radical doubt: can one doubt that God exists? The answer: no — because God enters into the very proof that the external world exists. Without God as guarantor we do not know whether an evil demon is deceiving us. It is a strange argument — and an honest one: rationalism runs up against questions it cannot close by itself.
Kant demolished all the classical proofs of God's existence — the ontological, the cosmological, the teleological. Reason cannot step beyond possible experience: God, the soul, freedom are ideas of reason, not objects of knowledge. It looks like the death of religion. But Kant said: I had to deny knowledge in order to make room for faith. God comes in not through theory — through the moral law. Everyone after him repeated that move.
If three great religions believe in one God — why is there so much blood between them?