The Transformation of Matter
Of Lanfranc's (c.1005-1089) many accomplishments, perhaps the most far-reaching was his writing on the subject of transubstantiation, the doctrine that the bread and wine in the Christian mass become the body and blood of Christ.
Not all authorities agreed on exactly what was meant in Matthew, Mark and Luke when Jesus offered bread and wine to the Apostles. St. Ignatius of Antioch, in a letter to the Romans, was very clear in 106 CE when he said "I desire the bread of GOD, which is the flesh of Jesus Christ." Paschasius Radbertus (785-865) wrote that the substance of the bread and wine was identical to the body and blood of Christ in heaven.
Although many early church fathers wrote in support of this, there was mild opposition for generations. A turning point came when Berengar of Tours (c. 999–January 6, 1088), a brilliant theologian, declared that the divinity imbued in the Eucharist did not rely on a change in its physical material. This was not radical enough to get him condemned--it acknowledged the special quality of the bread and wine--but Berengar wrote a letter to Lanfranc, chastising him for not rejecting Paschasius' view as well. The letter had to travel from Tours to Rome, where Lanfranc was at the time; by the time it reached Lanfranc, it had been read by several people, and the beginning of a public conflict was in the air. Lanfranc, knowing that Berengar's view was not looked upon favorably, and not wanting to prejudice the pope against Lanfranc's own future, took up the cause and wrote a public argument in opposition to Berengar. The world had a new and respected authority now for the transformation of the body and blood of Christ. The term transubstantiated was used to describe the change in the Fourth Lateran Council of 1215, and Protestant Christianity had a point of contention for centuries to come.
(Teaser: for me, the best piece of doctrinal explanation came from a student of Lanfranc, who also became Archbishop of Canterbury. I look forward some day to telling you about Anselm of Bec.)