(2) continued from above…
The Church from the very beginning also prescribed such acts of satisfaction. One common one, early on, was cutting the sinner off from the sacraments for a period of time (this is still common in the East and for certain more serious sins in the West). St. Paul prescribes such satisfaction for the incestuous man in Corinth here:
1 Cor. 5:[3] I indeed, absent in body, but present in spirit, have already judged, as though I were present, him that hath so done, [4] In the name of our Lord Jesus Christ, you being gathered together, and my spirit, with the power of our Lord Jesus; [5] To deliver such a one to Satan for the destruction of the flesh, that the spirit may be saved in the day of our Lord Jesus Christ.
Later, the Church adopted other forms of satisfaction in different times and places many taking months or even years to complete. For example, someone who committed a very grievous sin might have to live a strict disciplined life in a monastery for a period of time (this is why prisons are often called penitentiaries, from the root “penitent.”)
If someone were to die before bringing forth worthy fruits, that process was and is understood to be finished in the afterlife in what we call Purgatory. That’s what prayers and offerings for the dead helps remit and those have always been practiced.
An indulgence is a remittance of this need to make satisfaction either partially or fully (a "plenary indulgence) granted by the bishop by virtue of his power of binding and loosing. St. Paul granted such am indulgence to the same incestuous man in Corinth:
2 Cor. 2 :[6] To him who is such a one, this rebuke is sufficient, which is given by many: [7] So that on the contrary, you should rather forgive him and comfort him, lest perhaps such a one be swallowed up with overmuch sorrow. [8] Wherefore, I beseech you, that you would confirm your charity towards him. [9] For to this end also did I write, that I may know the experiment of you, whether you be obedient in all things. [10] And to whom you have pardoned any thing, I also. For, what I have pardoned, if I have pardoned any thing, for your sakes have I done it in the person of Christ.
This remitting of some or all of a prescribed satisfaction was common throughout the Church, in East and West. This shouldn’t be controversial. See the subsection “The power to grant indulgences” in this article for some specific examples in history from the early Church of these remittances being granted:
http://www.newadvent.org/cathen/07783a.htm
Anyway, since the need to make satisfaction extends into the afterlife, it followed logically that the indulgence would also. Otherwise, if one were loosed on earth, but it did not take effect in the afterlife, the indulgence would be a cruelty deceiving the penitent into thinking he had brought forth sufficient fruits when he had not.
In later times in the West, continuing until today, since prescribed penances are generally meant to simply turn the penitent back to God through prayer rather than equal the total fruits worthy of repentance, the focus of indulgences is more on their effects in the afterlife.
continued…