Question: What are indulgences? In various books I have seen mention made of indulgences for various prayers. Often they speak of "quarantines"; other times they speak of days, months or years. Please explain the way indulgences are measured.
Question: If the Church can grant indulgences to the dead, why doesn't It just release everyone from Purgatory?
Answer: An indulgence is an act of the Church by which It pardons a person (who has confessed his sins and received absolution) from the temporal punishment due to sin. It does not forgive sins and cannot be received by those in the state of serious sin. Even after going to Confession, we are still bound in justice to pay the penalty for the moral damage caused by our sins. Our ability to satisfy this temporal punishment through our own good works is relatively limited. The Church, however, is able to apply the super-abundant merits of our Lord and His saints to Its subjects.
The words "super-abundant" remind us that the merits gained by our Lord on the Cross are infinite. Coupled with the good works and graces earned by the saints, they form an inexhaustible treasury of merit, more than able to make up for the temporal punishments due to all mankind. The Church is able to apply these merits to Its subjects by the "power of the keys," which enables Peter and the Apostles to "bind in heaven whatever they have bound in heaven" (Cf. Matthew xvi).
In the past, the Church granted indulgences mostly for services rendered to It by the faithful. In the middle ages the most common indulgences were earned by those contributing to the building of a church or participating in a crusade. In order to avoid the appearance of selling indulgences, which would constitute the sin of simony, the Church now generally restricts Itself to granting indulgences for pilgrimages and prayers, which do not monetarily benefit the Church or the clergy.
Prior to the reign of Pope Paul VI, indulgences were designated as "plenary," satisfying all of the temporal punishment due to sin; or as "partial," satisfying this punishment as though the recipient had performed the traditional Lenten penance for some number of days, weeks, months, or "quarantines." A quarantine is simply the length of time equal to an entire Lent.
It should be noted that the measurement indicates an equivalent penance, and not a reduction of a "sentence" to be "served" in Purgatory. We simply do not know how the duration of punishment is measured in Purgatory, nor do we know if there is time in Purgatory or how it might relate to time on earth. In any event, the primary application of indulgences is to the living, the subjects of the Church Militant. The Church on earth sometimes applies Its indulgences to the Faithful Departed, but does so not by authority but by way of suffrage, asking God to accept Its petition for pardon of the deceased.
Since the effectiveness of an indulgence gained for a soul in Purgatory is uncertain, we are able and even urged to gain numerous indulgences for the deceased. Such effort is not wasted, even if the person for whom the indulgence is gained is unable to receive its benefits. God never wastes anything, but employs such ungarnered merits for purposes He judges worthy.
In 1967, Pope Paul VI modified the basis for classifying indulgences. They remained either plenary or partial, but only one plenary indulgence may now be earned in any given day; and partial indulgences now have an indefinite value, not calibrated in terms of periods of penance but essentially matching the penitential value of the indulgenced act (a sort of spiritual "two for one"). Pope Paul's decree helps preclude the possibility of holding a "mechanical" attitude toward indulgences, eliminating things like multiple plenary indulgences for "visiting" the same church several times in a day by repeatedly walking in and out of it!
Prior to 1967, The Raccolta was the official catalog of authentic indulgences. The current enumeration is called The Enchiridion of Indulgences. The Church strictly prohibits the publication of unauthorized indulgences, and indulgences not properly authorized are null and void.
It can be seen from what was said above that the Church cannot "just release everyone from Purgatory." The Church Militant, that is the Church on earth, does not have, nor does it claim, jurisdiction over the souls in Purgatory. But it does generously give the living abundant opportunities to earn indulgences and offer them on behalf of the deceased. The Church Itself, "extremely solicitous for the faithful departed, has decided to apply suffrages to them as abundantly as possible in every Sacrifice of the Mass" (Enchiridion, "Norms on Indulgences," No 21).
Taking this cue from the Church, we ought to make a point to offer our prayers and good works, and particularly our participation at Holy Mass and reception of Holy Communion for those who have gone before us. If we forget to pray for the Souls in Purgatory when they have need of us, they may forget to pray for us when we need them!