The City of God

And hence that true Mediator, in so far as, by assuming the form of a servant, He became the Mediator between God and men, the man Christ Jesus, though in the form of God He received sacrifice together with the Father, with whom He is one God, yet in the form of a servant He chose rather to be than to receive a sacrifice, that not even by this instance any one might have occasion to suppose that sacrifice should be rendered to any creature. Thus He is both the Priest who offers and the Sacrifice offered. And He designed that there should be a daily sign of this in the sacrifice of the Church, which, being His body, learns to offer herself through Him. Of this true Sacrifice the ancient sacrifices of the saints were the various and numerous signs; and it was thus variously figured, just as one thing is signified by a variety of words, that there may be less weariness when we speak of it much. To this supreme and true sacrifice all false sacrifices have given place.