Leviticus 9

It seems that they have not paid sufficient attention to the fact that Zachary was a priest and that all priests at that time were obliged by the law of God to offer sacrifice first for their own sins and then for those of the people. Therefore, as it is now proved by the sacrifice of prayer that we are not sinless, since we are [daily] commanded to say, "Forgive us our debts," so it was proved then by the sacrifice of animal victims that the priests were not sinless, since they were commanded to offer the victim for their own sins.

Something of this sort has also been written about Aaron in the book of Leviticus, namely, that "he raised up his hands over the people and blessed them." I gather from this that the saying expresses a mystery, namely, that the one who blesses someone must be adorned with works that distinguish him from the many and raise him above them. For when he is going to bless the people, the hands of Aaron are raised on high. So if someone has his hands down toward earthly things, he does not intend to bless anyone.