|Early copy of the Sentences|
Peter Lombard (c.1095-1160) was born in Italy. He studied at Reims and Paris, and taught for ten years in the cathedral school at Notre Dame where he would have met some of the greatest theologians of the time. He was ordained by 1156, and was made bishop of Paris in 1159.
At some point he found time to write. Although he wrote commentaries of his own, his great work was the Libri Quatuor Sententiarum (Four Books of Sentences), in which he attempted to cover the entirety of biblical scholarship and knowledge by laying out passages from the Bible with relevant commentary from the Early Church Fathers and others. He tries to show where there is agreement among the commentators; where there is disagreement, he tries to reconcile the opposing viewpoints.
The four books covered the Trinity, Creation (and the world), Christ and salvation, and the Sacraments. The Sentences became the standard theology textbook for the next 400 years, and formed the basis for understanding the Bible and Christianity.
Because of the fundamental position the Sentences takes in theology, it was at the center of most theological study. Therefore, students were given a simple task: write your own commentary on the Sentences of Peter Lombard. Your masters would read your commentary to see if you understood the Bible and its learned analyses properly. If you did not, you were told to correct your commentary. If you corrected your commentary, all was well and good. If you decided that you were right and that you should argue with your masters, you ran the risk of (at the very least) not having your degree conferred, or (at worst) being declared heretical.
...and that's what one of the most famous medieval thinkers—the one whose name everyone today knows—did, and it got him expelled from Oxford University.
But that's a story for another day.