A reader writes,
"I would really like you to address this whole issue of Plenary Indulgences. I would like to believe in them but I really struggle with them. The fact that I make so many first friday masses I will reduce my time in purgatory or if I do a divine mercy chaplet from Good Friday to Easter Sunday will relieve me of all my sins during that time just doesn't sit right. Like I said, I want to believe this but I am really struggling with it. Please clarify this whole issue if you would."
First off, it is important to understand the nature of sin, including the two consequences of sin. The first of these consequences, of course, is that we have offended God, and we must obtain forgiveness from him. In the case of mortal sins, this can only happen in the sacrament of reconciliation, which Jesus Christ established. Those who believe otherwise are kidding themselves. If we do not reconcile with God, there is an eternal consequence, which is eternity in Hell.
Secondly, however, we have caused temporal damage to our relationships with one another and with God (and with ourselves). Our goal in life, the definition of spiritual maturity, is to completely break any hold that worldly attachments have on us and to turn one-hundred percent to God. However, every time we sin, even in a minor way, we give a part or ourselves to something "of the flesh". God has forgiven us, but we have given part of ourselves over to something that is a distraction to him, whether it be impure thoughts, greed, or laziness, etc.
I sometimes use the analogy of a wedding. Imagine a young couple at the altar. Though the wife promises to love the husband, she goes through the entire ceremony thinking about her past boyfriends. In our wedding feast with God, he does not want us distracted by thoughts of our past "loves", those sins we turned to time and time again.
This is why, when we sin, not only do we need forgiveness, we need to "drive and train" our body to reject those sins in the future, and this comes in the form of penance or, if we die before accomplishing this, Purgatory. When we sin once, it so damages us spiritually that it is easier to sin again, so we must consciously chose righteous acts (such as the penance of three Hail Mary and two Our Father prayers the priest might assign us) to recalibrate our conscience. Think of it this way, if we fell into a bad habit of eating too much unhealthy food, we would need a lifestyle change of diet and exercise to correct this, and like penance, it might be uncomfortable sometimes.
Indulgences come as a result of us choosing righteous acts, such as prayer and Scripture study. Because such acts are a sign of turning toward God, he recognizes our attempts to detach from the temporal desires, so he relieves us of the temporal consequences of our actions.
A lot of people think that indulgences are an "abuse" of the church from the time of Luther, but this is incorrect. What happened during this period was that, rightfully so, indulgences were granted for alms-giving. After all, if someone gives to the poor, using money that he could spend on selfish desires and for which he worked hard to obtain, isn't this a good and righteous act? However, one can see how abuses could creep in, which is what happened in this time period.
Another misconception, as mentioned in the question, is that indulgences knock off so many "years" of Purgatory. First off, we do not know enough about Purgatory to even say if actual years are involved. So, when the Church speaks of relieving X number of years from Purgatory, this is saying that one can relieve, through an indulgence, the temporal punishment in Purgatory that would be equal to that many years of penance during earthly life.
One last thing worth mentioning on indulgences is that, as a member of the body of Christ, our sin affects all the parts of that body, which is why we are offending more than just God when we sin (there is no such thing as "private" sin) and another reason we confess to a priest, who stands also as a representative, not just of God, but of the entire human race. Likewise, when we chose righteous acts, God can reward us from the "treasury of satisfactions", which have been realized by the great saints in our Church's history, and he effects this through the power to bind and loosen given to the Church.
There is much to be said about indulgences, which are an infallible teaching of the Church, which means that Catholics are bound to believe in them. For the sake of keeping this post reasonable, I would like to refer readers to some great articles on the subject which have appeared in This Rock, a magazine put out by Catholic Answers.
Primer on indulgences
Myths About Indulgences
How to Gain an Indulgence