And the LORD was very angry with Aaron and would have destroyed him: and I prayed for Aaron also the same time.
All Commentaries on Deuteronomy 9:20 Go To Deuteronomy 9
Cyril of Jerusalem
AD 386
It was not the people alone that sinned but also Aaron, the high priest. For Moses says, “And the wrath of the Lord was upon Aaron”; “and I prayed for him,” he says, “and God forgave him.” Now Moses made supplication on behalf of the high priest who sinned and prevailed upon the Lord by his importunity. If so, will not Jesus, his onlybegotten Son, imploring God in our behalf, more so prevail? If he did not prevent Aaron, because of his falling away, from acceding to the high priesthood, can it be that he will prevent you, coming from paganism, from attaining salvation? Catechetical Lecture