Priests appointed through the law under the Old Covenant always went into the tabernacle to offer the blood of animals for their own sins and the sins of the people. It was a temporary solution that needed to constantly be repeated because these gifts and sacrifices could not meet God’s requirement of perfection, and left the worshippers in a state of sin consciousness.
But when Christ appeared as High Priest (after the law), everything radically changed.
In order for Jesus to become a permanent High Priest and for the New Covenant to be established, first the entire law of commandments which came through Moses had to be annulled and brought to an end. This promise or word of the oath occurred “after the law,” where the Son was made perfect forever.
The old priesthood and commandments from the law were defined as weak and useless by the writer of Hebrews—and couldn’t bring the required perfection for the people. This resulted in a better hope and a better covenant than what the Israelites had been under since they had escaped from Egypt. Jesus is that New Covenant of grace and we have been brought into Him.