Answer: The use of reason to support belief.
During the "Church Age" as it is sometimes called, Christianity dominated culture in Europe. Theology was the most highly-regarded subject in the universities which were founded during the Middle Ages, but medieval Christian scholars also pursued much philosophy and rhetoric -- the patterns of reasonable argument.
The medieval Christian scholars sought to use the tools of reason to demonstrate the truths that were shown in the Bible. They were careful to point out that they weren't trying to make religious belief into something that had to "make sense" in order to be believable. They accepted the Bible as truth from God. But they sought to understand matters of faith better by the use of reasonable thought.
Anselm of Canterbury (1033-1109) described his path as a man of both faith and reason this way: "I do not seek to understand in order that I may believe, but I believe in order to understand. For this also I believe--that unless I believe I shall not understand." Another prominent medieval Catholic scholar, Thomas Aquinas (1225-1274), said something similar: "To one who has faith, no explanation is necessary. To one without faith, no explanation is possible."