Objection 1. It would seem that indulgences are not as effective as they claim to be. For indulgences have no effect save from the power of the keys. Now by the power of the keys, he who has that power can only remit some fixed part of the punishment due for sin, after taking into account the measure of the sin and of the penitent's sorrow. Since then indulgences depend on the mere will of the grantor, it seems that they are not as effective as they claim to be.
Objection 2. Further, the debt of punishment keeps man back from the attainment of glory, which he ought to desire above all things. Now, if indulgences are as effective as they claim to be, a man by setting himself to gain indulgences might become immune from all debt of temporal punishment. Therefore it would seem that a man ought to put aside all other kinds of works, and devote himself to gain indulgences.
Objection 3. Further, sometimes an indulgence whereby a man is remitted a third part of the punishment due for his sins is granted if he contribute towards the erection of a certain building. If, therefore, indulgences produce the effect which is claimed for them, he who gives a penny, and then another, and then again another, would obtain a plenary absolution from all punishment due for his sins, which seems absurd.
Objection 4. Further, sometimes an indulgence is granted, so that for visiting a church a man obtains a seven years' remission. If, then, an indulgence avails as much as is claimed for it a man who lives near that church, or the clergy attached thereto who go there every day, obtain as much indulgence as one who comes from a distance (which would appear unjust); moreover, seemingly, they would gain the indulgence several times a day, since they go there repeatedly.
Objection 5. Further, to remit a man's punishment beyond a just estimate seems to amount to the same as to remit it without reason; because in so far as he exceeds that estimate, he limits the compensation. Now he who grants an indulgence cannot without cause remit a man's punishment either wholly or partly, even though the Pope were to say to anyone: "I remit to all the punishment you owe for your sins." Therefore it seems that he cannot remit anything beyond the just estimate. Now indulgences are often published which exceed that just estimate. Therefore they do not avail as much as is claimed for them.
On the contrary, It is written (Job 13:7): "Hath God any need of your lie, that you should speak deceitfully for Him?" Therefore the Church, in publishing indulgences, does not lie; and so they avail as much as is claimed for them.
Further, the Apostle says (1 Corinthians 15:14): "If... our preaching is vain, your faith is also vain." Therefore whoever utters a falsehood in preaching, so far as he is concerned, makes faith void. and so sins mortally. If therefore indulgences are not as effective as they claim to be, all who publish indulgences would commit a mortal sin: which is absurd.
I answer that, on this point there are many opinions. For some maintain that indulgences have not the efficacy claimed for them, but that they simply avail each individual in proportion to his faith and devotion. And consequently those who maintain this, say that the Church publishes her indulgences in such a way as, by a kind of pious fraud, to induce men to do well, just as a mother entices her child to walk by holding out an apple. But this seems a very dangerous assertion to make. For as Augustine states (Ep. ad Hieron. lxxviii), "if any error were discovered in Holy Writ, the authority of Holy Writ would perish." In like manner, if any error were to be found in the Church's preaching, her doctrine would have no authority in settling questions of faith.
Hence others have maintained that indulgences avail as much as is claimed for them, according to a just estimate, not of him who grants it--who perhaps puts too high a value on it--nor of the recipient--for he may prize too highly the gift he receives, but a just estimate according to the estimate of good men who consider the condition of the person affected, and the utility and needs of the Church, for the Church's needs are greater at one time than at another. Yet, neither, seemingly, can this opinion stand. First, because in that case indulgences would no longer be a remission, but rather a mere commutation. Moreover the preaching of the Church would not be excused from untruth, since, at times, indulgences are granted far in excess of the requirements of this just estimate, taking into consideration all the aforesaid conditions, as, for example, when the Pope granted to anyone who visited a certain church, an indulgence of seven years, which indulgence was granted by Blessed Gregory for the Roman Stations.
Hence others say that the quantity of remission accorded in an indulgence is not to be measured by the devotion of the recipient, as the first opinion suggested, nor according to the quantity of what is given, as the second opinion held; but according to the cause for which the indulgence is granted, and according to which a person is held deserving of obtaining such an indulgence. Thus according as a man approached near to that cause, so would he obtain remission in whole or in part. But neither will this explain the custom of the Church, who assigns, now a greater, now a lesser indulgence, for the same cause: thus, under the same circumstances, now a year's indulgence, now one of only forty days, according to the graciousness of the Pope, who grants the indulgence, is granted to those who visit a church. Wherefore the amount of the remission granted by the indulgence is not to be measured by the cause for which a person is worthy of an indulgence.
We must therefore say otherwise that the quantity of an effect is proportionate to the quantity of the cause. Now the cause of the remission of punishment effected by indulgences is no other than the abundance of the Church's merits, and this abundance suffices for the remission of all punishment. The effective cause of the remission is not the devotion, or toil, or gift of the recipient; nor, again, is it the cause for which the indulgence was granted. We cannot, then, estimate the quantity of the remission by any of the foregoing, but solely by the merits of the Church--and these are always superabundant. Consequently, according as these merits are applied to a person so does he obtain remission. That they should be so applied demands, firstly, authority to dispense this treasure. secondly, union between the recipient and Him Who merited it--and this is brought about by charity; thirdly, there is required a reason for so dispensing this treasury, so that the intention, namely, of those who wrought these meritorious works is safeguarded, since they did them for the honor of God and for the good of the Church in general. Hence whenever the cause assigned tends to the good of the Church and the honor of God, there is sufficient reason for granting an indulgence.
Hence, according to others, indulgences have precisely the efficacy claimed for them, provided that he who grants them have the authority, that the recipient have charity, and that, as regards the cause, there be piety which includes the honor of God and the profit of our neighbor. Nor in this view have we "too great a market of the Divine mercy" [St. Bonaventure, Sent. iv, D, 20, as some maintain, nor again does it derogate from Divine justice, for no punishment is remitted, but the punishment of one is imputed to another.
Reply to Objection 1. As stated above (Question 19, Article 3) there are two keys, the key of orders and the key of jurisdiction. The key of orders is a sacramental: and as the effects of the sacraments are fixed, not by men but by God, the priest cannot decide in the tribunal of confession how much shall be remitted by means of the key of orders from the punishment due; it is God Who appoints the amount to be remitted. On the other hand the key of jurisdiction is not something sacramental, and its effect depends on a man's decision. The remission granted through indulgences is the effect of this key, since it does not belong to the dispensation of the sacraments, but to the distribution of the common property of the Church: hence it is that legates, even though they be not priests, can grant indulgences. Consequently the decision of how much punishment is to be remitted by an indulgence depends on the will of the one who grants that indulgence. If, however, he remits punishment without sufficient reason, so that men are enticed to substitute mere nothings, as it were, for works of penance, he sins by granting such indulgences, although the indulgence is gained fully.
Reply to Objection 2. Although indulgences avail much for the remission of punishment, yet works of satisfaction are more meritorious in respect of the essential reward, which infinitely transcends the remission of temporal punishment.
Reply to Objection 3. When an indulgence is granted in a general way to anyone that helps towards the building of a church, we must understand this to mean a help proportionate to the giver: and in so far as he approaches to this, he will gain the indulgence more or less fully. Consequently a poor man by giving one penny would gain the full indulgence, not so a rich man, whom it would not become to give so little to so holy and profitable a work; Just as a king would not be said to help a man if he gave him an "obol."
Reply to Objection 4. A person who lives near the church, and the priest and clergy of the church, gain the indulgence as much as those who come perhaps a distance of a thousand days' journey: because the remission, as stated above, is proportionate, not to the toil, but to the merits which are applied. Yet he who toils most gains most merit. This, however, is to be understood of those cases in which an indulgence is given in an undeterminate manner. For sometimes a distinction is expressed: thus the Pope at the time of general absolution grants an indulgence of five years to those who come from across the seas, and one of three years to those who come from across the mountains, to others an indulgence of one year. Nor does a person gain the indulgence each time he visits the church during the term of indulgence, because sometimes it is granted for a fixed time; thus when it is said, "Whoever visits such and such a church until such and such a day, shall gain so much indulgence," we must understand that it can be gained only once. on the other hand if there be a continual indulgence in a certain church, as the indulgence of forty days to be gained in the church of the Blessed Peter, then a person gains the indulgence as often as he visits the church.
Reply to Objection 5. An indulgence requires a cause, not as a measure of the remission of punishment, but in order that the intention of those whose merits are applied, may reach to this particular individual. Now one person's good is applied to another in two ways: first, by charity; and in this way, even without indulgences, a person shares in all the good deeds done, provided he have charity: secondly, by the intention of the person who does the good action; and in this way, provided there be a lawful cause, the intention of a person who has done something for the profit of the Church, may reach to some individual through indulgences.
Source: Summa Theologica (New Advent)