Officially it was a part of the Lord’s Supper, but objectively it was and is the foundation of all sacraments, namely the power the priest has to produce God, facere deum – making God out of the bread and wine is the fundamental power of the Church in the Middle Ages.

Let me add one last word: There was one sacrament which was in a kind of tension with all the others, namely penance. Penance was the sacrament of personal piety and there was much discussion about it: What are the conditions of the forgiveness of sins in the sacrament of penance? Some made it very easy, some more heavy. All believed that a personal repentance is necessary – light or heavy and, on the other hand, that a sacrament is necessary. But how the sacrament and the personal element were related to each other, to this no Scholastic gave an answer; and this was the point in which the medieval Church exploded, by the intensification of the subjective side in the sacrament of penance. This was the experience of Luther, and therefore he became the reformer of the Church.
Paul Tillich, A History Of Christian Thought – Table of Contents