 So when he would go into the Holy of Holies, he would take the blood, apply it to the mercy seat, not only for the people, but first for himself. Because why? The high priest, being a man, was a sinner. All of sin comes short of the glory of God. The priest, being even the priest, the high priest, the one chosen by God, still had to have a covering for his sins. And he would do this year after year after year.