I would say that the simple thought of purgatory diminishes the power of the purification of the blood of Jesus. It is simply saying that Jesus' blood is not enough. Therefore you must go through another purification process in purgatory.
As far as salvation and purification. The death on the cross and the resurrection.
Well, that's not very specific but I guess it's good enough.
With respect, my gripe with certain types of Protestantism is that it is simplistic to the point of being reductionist. You sin, you ask forgiveness, you move on, that's the end of it.
Um, no. That's not the end of it.
If a small boy smashes out his neighbor's window, the neighbor can decide to forgive. And is that the end of it? The boy has forgiveness. Can he move on?
No, the window is still smashed out. What happens with that?
So the boy agrees to mow lawns all summer until he's saved up enough money to pay the neighbor back for the cost of the window. Is it over now?
No, of course not. How does the neighbor know the boy won't do the same thing again later on? The boy genuinely promises from his heart that he won't do this again. So
NOW it's settled.
Sin is analogous to that. If you sin, you can seek forgiveness from God. And He is gracious and just to forgive.
But that's not the end of the story. Logically it can't be. There's still the effects of your sin. Sure, you've been forgiven for sinning. But the harm you caused lingers.
Now, in life you can address this with Confession and acts of penitence to purify your soul. You've already been forgiven. But there are disciplines (purifications one might say) to be addressed.
However, Purgatory is the means by which this is addressed after death. As the Catechism would have it...
Catechism of the Catholic Church said:
The punishments of sin
1472 To understand this doctrine and practice of the Church, it is necessary to understand that sin has a double consequence. Grave sin deprives us of communion with God and therefore makes us incapable of eternal life, the privation of which is called the "eternal punishment" of sin. On the other hand every sin, even venial, entails an unhealthy attachment to creatures, which must be purified either here on earth, or after death in the state called Purgatory. This purification frees one from what is called the "temporal punishment" of sin. These two punishments must not be conceived of as a kind of vengeance inflicted by God from without, but as following from the very nature of sin. A conversion which proceeds from a fervent charity can attain the complete purification of the sinner in such a way that no punishment would remain.
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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.
1031 The 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.
So contrary to common misconception, Purgatory is not a "second chance". Eternal forgiveness is already assured. It's simply the means by which impurities are
purged. If the Church had called this "Purgation" as opposed to "Purgatory", I think it would be better understood by non-Catholics. Would've, should've, could've, I guess.
Still, point is that this mortal must put on immortality. This corruptible must be made incorruptible. Purgatory.