“What saved rationality after the Greeks gave it up?” Vishal Mangalwadi asks that question and answers it at length in his wide-ranging The Book That Made Your World,. He says,

It was the Bible's teaching that eternal life was to know God and Jesus Christ. That Jesus was someone in who were hidden the treasures of wisdome and knowledge. An entire section of the Bible is called Wisdom Literature an teaches that wisdom and understanding are far more important thatn rubies and diamonds.

The Bible, the Greeks, and Knowledge

There is an astonishing misconception out there that the “Dark Ages” came about because of Christian antipathy to knowledge. Nothing could be further from reality. Perhaps the chief cause for civilization's slowdown in Europe was the fall of Rome to barbarian military conquest, but as Mangalwadi explains, the Greeks had become “suspicious of logic” long before then. Sophist rhetoric made it seem more intended for manipulating belief than for knowing truth. Skepticism arose and flourished.

But the book of John introduces Jesus as the logos, the Word of God (John 1:1). I've found in my own study that words relating to knowing, studying, and learning occur an average of twice in every chapter of the New Testament. It could hardly escape notice (except it may be too obvious to see!) that God gave his primary revelation to us in the form of a book. God's word calls on us to be transformed by the renewing of our minds (Romans 12:1,2) and to love God with all our minds (Matt. 22:37-29).

St. Augustine Advancing Knowledge

And so it was that St. Augustine (AD 354-430) and the St. Boethius (ca. AD 480-524) “played the most important roles in preserving logic and laying hte intellectual foundations of medieval and modern Western civilization,” according to Mangalwadi.

Augustine saved the intellect from the skeptics' attack because he understood the biblical revelation to imply that our minds were God's most precious gift to us. They enabled us to be God's own image, to know him, and to love him.

He wrote prolifically himself, and exercised such influence that some still consider him the most important Christian thinker since the apostle Paul.

Boethius Preserving and Advancing Knowledge

Boethius's book The Consolation of Philosophy has likewise been influential for centuries. Mangalwadi tells us that he

translated philosophical, medical, and scientific works from Greek into Latin and also wrote philosophical and theological treatises. He inspired medieval scholars to continue developing their philosophical tradition until the “dawn” of the early Renaissance and Reformation.

Mangalwadi quotes historian Edward Grant,

Boethius guaranteed that logic, the most visible symbol of reason and rationality, remained alive at the lowest ebb of European civilization, between the fifth and tenth centuries.

John of Damascus Encouraging Knowledge

John of Damascus (AD 676-749), one of the late Church Fathers, taught that

to be spiritual was to cultivate the life of the mind…. John's work The Fount of Knowledge reinforced the belief that orthodox, biblical Christianity was a religion of rationality…. This is how he opened his first treatise: “Nothing is more estimable than knowledge, for knowledge is the light of the rational soul.”

These are but a few early examples of Christianity preserving and advancing knowledge. The Church has always been made of humans, and it would be false to oversell its intellectualism and rationality down through the centuries. It would be more false yet, however, to hold it responsible for holding back the progress of knowledge. Whatever might have been holding it back, men and women of the Church held it up so it could move forth.