What Purgatory is not
Purgatory is not a second chance.
Purgatory is not a place where the soul can do anything to cleans himself; all cleansing is done by God alone.
Purgatory is not a “third option” for people who are “too good” for heaven but “too bad” for heaven; there are only two destinations for those who have died: heaven or hell.
What is Purgatory?
Catechism of the Catholic Church
1030 All who die in God’s grace and friendship, but still imperfectly purified, are indeed assured of their eternal salvation; but after death they undergo purification, so as to achieve the holiness necessary to enter the joy of heaven.
1031The Church gives the name Purgatory to this final purification of the elect, which is entirely different from the punishment of the damned.606 The Church formulated her doctrine of faith on Purgatory especially at the Councils of Florence and Trent. The tradition of the Church, by reference to certain texts of Scripture, speaks of a cleansing fire:607
As for certain lesser faults, we must believe that, before the Final Judgment, there is a purifying fire. He who is truth says that whoever utters blasphemy against the Holy Spirit will be pardoned neither in this age nor in the age to come. From this sentence we understand that certain offenses can be forgiven in this age, but certain others in the age to come.608
1032This teaching is also based on the practice of prayer for the dead, already mentioned in Sacred Scripture: "Therefore [Judas Maccabeus] made atonement for the dead, that they might be delivered from their sin."609 From the beginning the Church has honored the memory of the dead and offered prayers in suffrage for them, above all the Eucharistic sacrifice, so that, thus purified, they may attain the beatific vision of God.610 The Church also commends almsgiving, indulgences, and works of penance undertaken on behalf of the dead:
Let us help and commemorate them. If Job’s sons were purified by their father’s sacrifice, why would we doubt that our offerings for the dead bring them some consolation? Let us not hesitate to help those who have died and to offer our prayers for them.611
Temporal effects of sin illustrated from scripture.
The temporal effects of sin are best illustrated in Genesis 3:14-20, where God pronounces a series of maledictions against Adam and Eve, the serpent, and even the physical creation itself, all as a result of Adam and Eve’s disobedience: the original sin. The eternal penalty of their sin—hell—was something Christ atoned for on the cross, and Adam and Eve’s salvation and avoidance of that penalty was due to their faith in God’s promises and their subsequent obedience to His laws, but the temporal effects of sin remained. Enmity with the rest of nature, having to work to eat, labor pains for women, sickness, and eventually death, were all part of the punishments God permitted to fall upon Adam and Eve and all of us, their descendants. So when you, a born-again Christian, receive Christ as your personal Lord and Savior, you correctly believe that He will forgive you of your sins and remit the eternal punishment they deserve. But you still are going to have to get up and go to work tomorrow, right? Your wife will still experience pain in childbirth; you may even someday be afflicted with cancer, or some other life-threatening disease. You might have a heart attack. And in the end you, like me and everybody else, will eventually suffer the greatest of the temporal punishments outlined in Genesis 3: You are going to die someday. Christ’s atoning sacrifice paid for your sins, but it didn’t wipe out the temporal effects, including the punishments I just mentioned of your sins.
There are other examples of the principle of temporal punishments due to sin that God does not even eliminate even when we repent and are forgiven of our sins.
For example, in 2 Samuel 12:14-14, we see the tragic case of King David. He had sinned by committing adultery and murder, and though the Lord forgave him when he repented, there were repercussions that followed which god did not alleviate. A form of restitution was still owed:
2 Samuel 12:13-14
13 David said to Nathan, “I have sinned against the LORD.” And Nathan said to David, “The LORD also has put away your sin; you shall not die. 14 Nevertheless, because by this deed you have utterly scorned the LORD, the child that is born to you shall die.”
Now, how does all this fit into the doctrine of purgatory? Purgatory is the place or state (however you’d like to think of it) in which the fire of God’s love purifies us from the temporal effects due to sin, which include any temporal punishments due to our sins.