Adam Clarke's Commentary on the Bible

Verse 21 (Hebrews 10:21)

A high priest over the house of God - The house or family of God is the Christian Church, or all true believers in the Lord Jesus. Over this Church, house, or family, Christ is the High Priest - in their behalf he offers his own blood, and their prayers and praises; and as the high priest had the ordering of all things that appertained to the house and worship of God, so has Christ in the government of his Church. This government he never gave into other hands. As none can govern and preserve the world but God, so none can govern and save the Church but the Lord Jesus: He is over the house; He is its President; he instructs, protects, guides, feeds, defends, and saves the flock. Those who have such a President may well have confidence; for with him is the fountain of life, and he has all power in the heavens and in the earth.

- Adam Clarke's Commentary on the Bible