Chapter 19 — Part II: The Sacraments — Penance
Section 101. Proof Of The Obligation — This truth is clearly conveyed by our Lord Himself, when, by a most beautiful metaphor, He calls the power of administering this Sacrament, the key of the kingdom of heaven. Just as no one can enter any place without the help of him who has the keys, so no one is admitted to heaven unless its gates be unlocked by the priests to whose custody the Lord gave the keys. This power would otherwise be of no use in the Church. If heaven can be entered without the power of the keys, in vain would they to whom the keys were given seek to prevent entrance within its portals.