Hebrews 5:1, 3, 6, 10, 12

For every high priest chosen from among men is appointed to act on behalf of men in relation to God, to offer gifts and sacrifices for sins.

Because of this he is obligated to offer sacrifice for his own sins just as he does for those of the people.

as he says also in another place, “You are a priest forever, after the order of Melchizedek.”

being designated by God a high priest after the order of Melchizedek.

For though by this time you ought to be teachers, you need someone to teach you again the basic principles of the oracles of God. You need milk, not solid food