The judicious Matthew Henry on Leviticus 16:2:
And the Lord said unto Moses, Speak unto Aaron thy brother, that he come not at all times into the holy place within the vail before the mercy seat, which is upon the ark; that he die not: for I will appear in the cloud upon the mercy seat.
That place where God manifested his special presence must not be made common. If none must come into the presence-chamber of an earthly king uncalled, no, not the queen herself, upon pain of death (Esther 4:11), was it not requisite that the same sacred respect should be paid to the Kings of kings? But see what a blessed change is made by the gospel of Christ; all good Christians have now boldness to enter into the holiest, through the veil, every day (Hebrews 10:19, 20); and we come boldly (not as Aaron must, with fear and trembling) to the throne of grace, or mercy-seat (Hebrews 4:16). While the manifestations of God's presence and grace were sensible, it was requisite that they should thus be confined and upon reserve, because the objects of sense the more familiar they are made the less awful or delightful they become; but now that they are purely spiritual it is otherwise, for the objects of faith the more they are conversed with the more do they manifest of their greatness and goodness: now therefore we are welcome to come at all times into the holy place not made with hands, for we are made to sit together with Christ in heavenly places by faith (Ephesians 2:6). Then Aaron must not come near at all times, lest he die; we now must come near at all times that we may live: it is distance only that is our death. Then God appeared in the cloud upon the mercy-seat, but now with open face we behold, not in a dark cloud, but in a clear glass, the glory of the Lord, (2 Corinthians 3:18).