Is Purgatory a part of Divine Revelation? What other things are considered Divine Revelation? Just a thought that popped into my head.
What other things are considered Divine Revelation?
Everything that the Church teaches. It’s a long list. The Catechism of the Catholic Church defines, maybe, a tenth of it.
Divine revelation is what it sounds like––a revealing by God of Himself, most profoundly culminated in the person of Jesus Christ. Any truth flowing from Christ by way of Apostolic teaching on faith or morals is divine revelation, which would include Purgatory, which flows from that single “wellspring.” See for example:
There is also “private revelation” which would be something like a divine appearance or special message by God to an individual or small group of people, but I think you are referring to public/general revelation.
Jesus alludes to its existence in the Gospels (see e.g. Matt 12:32) and his audience knew what he was talking about.
2 Maccabees shows, for example, that the Jews already believed that some souls could die with sins on their conscience that could yet still possibly be forgiven by offering sacrifices to God. Jesus affirms this belief when he teaches that there is a certain sin (the sin against the Holy Spirit) that cannot be forgiven through such expiatory sacrifices known to the Jews which were made by them on behalf of the dead or for the souls of sinners after death (“forgiveness…in…the world to come” (Matt 12:32)).
The Church teaches in the Catechism that the aforesaid sin is the sin of final impenitence or an absolute refusal to acknowledge, cooperate with and accept God’s mercy and grace: it is in effect an ardent or obstinate refusal to be reconciled with God.
The early Christians all prayed for the dead. Again, this would be a futile, worthless and possibly even heretical practice if there simply was no way to console or provide assistance for the dearly departed: they would be either absolutely in heaven or hell for all eternity and entertaining any hope for them would be a violation of God’s just decree. However, if there does exist a “place” where souls are undergoing purification from sin -and this is God’s will- then it is not impious to hope that some souls have gone to that “place”.
Consequently, we can pray to God to have mercy on them or do penitential works (or works of charity) on their behalf to make up for whatever may be lacking on their part:
 Even as I write, I am glad of my sufferings on your behalf, as, in this mortal frame of mine, I help to pay off the debt which the afflictions of Christ still leave to be paid, for the sake of his body, the Church.
Jesus also alludes to a place of punishment for debtors (which is also a temporary one) in the Gospels:
 If any man has a claim against thee, come to terms there and then, while thou art walking in the road with him; or else it may be that the claimant will hand thee over to the judge, and the judge to the officer, and so thou wilt be cast into prison.
 Believe me, thou shalt not be set at liberty until thou hast paid the last farthing.
Notice that you can be released from this “prison” (v.25); that is, you can “be set at liberty” (v.26) but not, however, until you have “paid the last farthing” (v.26, “farthing,” i.e., the last penny as in the old debtor’s prisons where people had to repay your debt to get you released).
When Christians pray for the dead or do works of charity on their behalf, we are attempting to pay those pennies (“farthings”: i.e. their debt):
 I was a stranger, and you did not bring me home… I was sick and in prison, and you did not care for me.
 Whereupon they, in their turn, will answer, Lord, when was it that we saw thee… in prison, and did not minister to thee?
 And he will answer them, Believe me, when you refused it to one of the least of my brethren here, you refused it to me.
Purgatory, then, is a kind of debtor’s prison, which is why it is important to pay that debt so those prisoners -who are our and the Lord’s brothers- may be released.
 Remember those who are in prison, as if you were prisoners too; those who endure suffering, since you have mortal bodies of your own.
I might also add from the Old Testament a witness from another book (Job) that the Protestants also accept, which Saint John Chrysostom (if memory serves me) also referenced for defending prayers for the dead, which the Catechism also cites when talking about Purgatory or prayers for the dead:
 And this custom his sons had in feasting, each invited the rest in turn; at such times they would send for their three sisters to eat and drink beside them.
 And ever when their week of feasting was over, Job would send for them, and have them rid of all defilement; next morning, **it was his first care to offer burnt-sacrifice for each **of them. Who knows, thought he, but they may have committed some fault, these children of mine? Who knows but they may have slighted God in their secret thoughts? Never would he let the day pass without burnt-sacrifice.
In keeping with what Saint Paul said to the Colossians referenced in my above post (“I am glad of my sufferings on your behalf, as, in this mortal frame of mine, I help to pay off the debt which the afflictions of Christ still leave to be paid…”) we see Job here offering sacrifices for his children’s possible sins. In context there is no doubt that the author of Job is approving of this practice: it is being referenced, in fact, as a witness to Job’s righteousness.
So the belief that we can offer our sufferings and make sacrifices to God on behalf of others is a very old one that is well attested to throughout the whole Bible.