In the early ages of the Church notorious sinners, after being absolved, were sentenced to long public penances. By sincere sorrow, an indulgence or remission of some of the time was granted them. Public confession and public penances have passed away. These public penances are replaced by pious devotions. Upon the performance of certain pious devotions the Church at times grants an indulgence; that is, a remission of such temporal punishment as is equivalent to the canonical penances corresponding to the sins committed.
Attached to every mortal sin, besides the guilt, is the punishment incurred. This punishment is eternal and temporal. That there is this twofold punishment we learn from various places in the Bible. We have an example in the sin of David. God sent the prophet Nathan to warn him of his guilt. When Nathan rebuked the king, he confessed his sin with signs of true contrition. Then Nathan told him that God had forgiven his sin, but that many temporal punishments would follow. When God forgave the sin, the guilt and eternal punishment were taken away; but temporal punishment remained. Other examples could be cited, but this is sufficient to show that there is a twofold kind of punishment—eternal and temporal. In confession the guilt and eternal punishment are taken away, but not always the temporal punishment. This temporal punishment is what is taken away in whole by a plenary and in part by a partial indulgence.
In a similar manner we have a twofold punishment attached to crime in this world. A man commits a crime. He is sentenced to a term in the penitentiary. After spending his time of punishment he comes back to society, but finds he has another punishment to undergo in being avoided by his friends and others.
The practice of granting indulgences was founded on many passages of Scripture, both of the Old and New Testament. In the 12th chapter of the book of Numbers we learn that Mary, the sister of Moses, was forgiven a sin which she had committed. But God inflicted upon her the penalty of leprosy. This was a temporal punishment. By the prayer of Moses an indulgence was granted; for God took away the temporal punishment.
Our divine Lord left with His Church the power of granting indulgences, as we learn from His words taken from St. Matthew: "Whatsoever you shall loose upon earth shall be loosed also in heaven." This promise implies the power of loosing not only from sin and its eternal punishment, but also the power of releasing the bond of temporal punishment, of freeing from everything that would prevent the soul from entering the kingdom of heaven. St. Paul granted an indulgence to the incestuous Corinthian, as we learn from the 2d chapter of his Second Epistle to the Corinthians. By the power and authority which he received from Christ, he granted the Corinthian pardon from performing a certain penance. This penance was a temporal punishment. The apostle took away the temporal punishment. That is an indulgence.
Non-Catholics grant a kind of plenary indulgence to every one by saying that works of penance are unnecessary. The practice of the Catholic Church of granting an indulgence only to the deserving is certainly more conformable to Scripture as well as more reasonable.
Experience teaches us the utility of indulgences. They encourage the faithful to frequent the sacraments, to repent, to do acts of penance, and perform works of piety, charity, and devotion.
A practice productive of such beneficial results is reasonable; it is also reasonable because it is sanctioned by Scripture and the Church of every age. For God would not sanction it nor could the Church practise it if it were not conformable to reason.
[VIII. The Last Sacraments]
"Is any man sick among you? Let him bring in the priests of the Church, and let them pray over him, anointing him with oil in the name of the Lord: And the prayer of faith shall save the sick man, and the Lord shall raise him up, and if he be in sins they shall be forgiven him" (James v. 14, 15).