In this post I’d like to look at the logical case for purgatory. But before we do, let’s take a look at what purgatory is. The Catechism teaches:
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 . . . The Church gives the name Purgatory to this final purification of the elect, which is entirely different from the punishment of the damned.” (CCC 1030 – 1031).
Catholic Invention ?
Non-Catholic Christians “may be fond of saying the Catholic Church ‘invented’ the doctrine of purgatory to make money, but they have difficulty saying just when. Most professional anti-Catholics—the ones who make their living attacking “Romanism”—seem to place the blame on Pope Gregory the Great, who reigned from A.D. 590–604.
But that hardly accounts for the request of Monica, mother of Augustine, who asked her son, in the fourth century, to remember her soul in his Masses. This would make no sense if she thought her soul would not benefit from prayers, as would be the case if she were in hell or in the full glory of heaven.
Nor does ascribing the doctrine to Gregory explain the graffiti in the catacombs, where Christians during the persecutions of the first three centuries recorded prayers for the dead.
Indeed, some of the earliest Christian writings outside the New Testament, like the Acts of Paul and Thecla and the Martyrdom of Perpetua and Felicity (both written during the second century), refer to the Christian practice of praying for the dead. Such prayers would have been offered only if Christians believed in purgatory, even if they did not use that name for it. (See Catholic Answers’ Fathers Know Best tract The Existence of Purgatory for quotations from these and other early Christian sources.) 
The Argument for Purgatory
I would like to thank Jimmy Akin for introducing me to the logical argument for purgatory; it can be formulated as follows:
Premise 1: There will be neither sin nor attachment to sin in Heaven.
Premise 2: We (at least most of us) are still sinning and are attached to sin at the end of this life.
Conclusion: Therefore there must be a period between death and heavenly glory in which the saved are cleansed of sin and their attachment to sin.
Because this is a deductive argument, if one wants to dispute the conclusion, he must take issue with one of the premises, since the conclusion follows from them necessarily.
So which is it?
Is it not true that the saved in heaven are perfectly sanctified? (“[N]othing unclean shall enter [Heaven]” – Rev 21:27).
Or is it not true that we are still sinning and attached to sin at the end of our earthly life? (If we say we have no sin, we deceive ourselves, and the truth is not in us.” – 1 John 1:8).
You can’t get out of this argument by asserting, as I read one man say, that Christ covers us with his righteousness the moment we are justified and therefore sees us as he sees his Son, Jesus. Okay, well, are we to conclude from this that we really will be sinning in heaven while God pretends that we’re fully sanctified and clean? The answer, I think, is obviously not.
How Long Does Purgatory Take?
Because we don’t know how time works in the afterlife, different theologians have had different speculations as to how long purgatory may take or if it takes any time at all. Cardinal Joseph Ratzinger (now Pope Emeritus Benedict XVI) seemed to side with the latter opinion. He wrote:
The transforming ‘moment’ of this encounter cannot be quantified by the measurements of earthly time.
It is, indeed, not eternal but a transition, and yet trying to qualify it as of ‘short’ or ‘long’ duration on the basis of temporal measurements derived from physics would be naive and unproductive.
The ‘temporal measure’ of this encounter lies in the unsoundable depths of existence, in a passing-over where we are burned ere we are transformed.
To measure such Existenzzeit, such an ‘existential time,’ in terms of the time of this world would be to ignore the specificity of the human spirit in its simultaneous relationship with, and differentation from, the world.
. . .
“[Purgatory] is the inwardly necessary process of transformation in which a person becomes capable of Christ, capable of God and thus capable of unity with the whole communion of saints. [Read More]
I’m aware that I haven’t presented a Biblical defense of the doctrine of purgatory; that wasn’t my intention. If you’re looking for one, click here. Or If you’re looking for a great and thorough (originally several CD’s) mp3 on the subject, click here.