The Historical Doctrine of Purgatory 
The Catechism of the Catholic Church states, “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 1030–1).
The doctrine of purgatory, or the final purification, has been part of the true faith since before the time of Christ. The Jews already believed it before the coming of the Messiah, as revealed in the Old Testament (2 Macc. 12:41–45) as well as in other pre-Christian Jewish works. The concept of an after-death purification from sin and the consequences of sin is also stated in the New Testament in passages such as 1 Corinthians 3:11–15 and Matthew 5:25–26, 12:31–32. Orthodox Jews to this day believe in the final purification, and for eleven months after the death of a loved one, they pray a prayer called the Mourner’s Kaddish for their loved one’s purification.
It was not until the Protestant Reformation in the sixteenth century that anyone denied the doctrine of the final purification. Some imagine that the Catholic Church has an elaborate doctrine of purgatory worked out, but there are only three essential components of the doctrine. (1) A purification after death exists. (2) It involves some kind of pain. (3) The purification can be assisted by the prayers and offerings by the living to God.
The Acts of Paul and Thecla
And after the exhibition, Tryphaena again received her [Thecla]. For her daughter Falconilla had died, and said to her in a dream: “Mother, you shall have this stranger Thecla in my place, in order that she may pray concerning me, and that I may be transferred to the place of the righteous” (Acts of Paul and Thecla [A.D. 160]).
The Martyrdom of Perpetua and Felicity
That very night, this was shown to me in a vision: I [Perpetua] saw Dinocrates going out from a gloomy place, where also there were several others, and he was parched and very thirsty, with a filthy countenance and pallid color, and the wound on his face which he had when he died. This Dinocrates had been my brother after the flesh, seven years of age, who died miserably with disease. . . . For him I had made my prayer, and between him and me there was a large interval, so that neither of us could approach to the other . . . I made my prayer for my brother day and night, groaning and weeping that he might be granted to me. Then, on the day on which we remained in fetters, this was shown to me: I saw that the place which I had formerly observed to be in gloom was now bright; and Dinocrates, with a clean body well clad, was finding refreshment. . . . [And] he went away from the water to play joyously, after the manner of children, and I awoke. Then I understood that he was translated from the place of punishment (The Martyrdom of Perpetua and Felicity 2:3–4 [A.D. 202]).
Tertullian
That allegory of the Lord [Matt. 5:25–26] . . . is extremely clear and simple in its meaning . . . [beware lest as] a transgressor of your agreement, before God the judge . . . and lest this judge deliver you over to the angel who is to execute the sentence, and he commit you to the prison of hell, out of which there will be no dismissal until the smallest even of your delinquencies be paid off in the period before the resurrection. What can be a more fitting sense than this? What a truer interpretation?" (The Soul 35 [A.D. 210]).
Cyprian
It is one thing to stand for pardon, another thing to attain to glory; it is one thing, when cast into prison, not to go out thence until one has paid the uttermost farthing; another thing at once to receive the wages of faith and courage. It is one thing, tortured by long suffering for sins, to be cleansed and long purged by fire; another to have purged all sins by suffering. It is one thing, in fine, to be in suspense till the sentence of God at the day of judgment; another to be at once crowned by the Lord (Letters 51[55]:20 [A.D. 253]).
Lactantius
But also, when God will judge the just, it is likewise in fire that he will try them. At that time, they whose sins are uppermost, either because of their gravity or their number, will be drawn together by the fire and will be burned. Those, however, who have been imbued with full justice and maturity of virtue, will not feel that fire; for they have something of God in them which will repel and turn back the strength of the flame (Divine Institutes 7:21:6 [A.D. 307]).
Cyril
Then we make mention also of those who have already fallen asleep: first, the patriarchs, prophets, apostles, and martyrs, that through their prayers and supplications God would receive our petition; next, we make mention also of the holy fathers and bishops who have already fallen asleep, and, to put it simply, of all among us who have already fallen asleep. For we believe that it will be of very great benefit to the souls of those for whom the petition is carried up, while this holyand most solemn sacrifice is laid out" (Catechetical Lectures 23:5:9 [A.D. 350]).
Gregory of Nyssa
If a man distinguish in himself what is peculiarly human from that which is irrational, and if he be on the watch for a life of greater urbanity for himself, in this present life he will purify himself of any evil contracted, overcoming the irrational by reason. If he has inclined to the irrational pressure of the passions, using for the passions the cooperating hide of things irrational, he may afterward in a quite different manner be very much interested in what is better, when, after his departure out of the body, he gains knowledge of the difference between virtue and vice and finds that he is not able to partake of divinity until he has been purged of the filthy contagion in his soul by the purifying fire (Sermon on the Dead [A.D. 382]).
John Chrysostom
Let us help and commemorate them. If Job’s sons were purified by their father’s sacrifice [Job 1:5], 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 (Homilies on First Corinthians 41:5 [A.D. 392]).
Not in vain was it decreed by the apostles that in the awesome mysteries remembrance should be made of the departed. They knew that here there was much gain for them, much benefit. When the entire people stands with hands uplifted, a priestly assembly, and that awesome sacrificial victim is laid out, how, when we are calling upon God, should we not succeed in their defense? But this is done for those who have departed in the faith, while even the catechumens are not reckoned as worthy of this consolation, but are deprived of every means of assistance except one. And what is that? We may give alms to the poor on their behalf (Homilies on Philippians 3:9–10 [A.D. 402]).
Augustine
There is an ecclesiastical discipline, as the faithful know, when the names of the martyrs are read aloud in that place at the altar of God, where prayer is not offered for them. Prayer, however, is offered for other dead who are remembered. It is wrong to pray for a martyr, to whose prayers we ought ourselves be commended (Sermons 159:1 [A.D. 411]).
But by the prayers of the holy Church, and by the salvific sacrifice, and by the alms which are given for their spirits, there is no doubt that the dead are aided, that the Lord might deal more mercifully with them than their sins would deserve. The whole Church observes this practice which was handed down by the Fathers: that it prays for those who have died in the communion of the body and blood of Christ, when they are commemorated in their own place in the sacrifice itself; and the sacrifice is offered also in memory of them, on their behalf. If, then, works of mercy are celebrated for the sake of those who are being remembered, who would hesitate to recommend them, on whose behalf prayers to God are not offered in vain? It is not at all to be doubted that such prayers are of profit to the dead (ibid., 172:2).
Temporal punishments are suffered by some in this life only, by some after death, by some both here and hereafter, but all of them before that last and strictest judgment. But not all who suffer temporal punishments after death will come to eternal punishments, which are to follow after that judgment (The City of God 21:13 [A.D. 419]).
That there should be some fire even after this life is not incredible, and it can be inquired into and either be discovered or left hidden whether some of the faithful may be saved, some more slowly and some more quickly in the greater or lesser degree in which they loved the good things that perish, through a certain purgatorial fire (Handbook on Faith, Hope, and Charity 18:69 [A.D. 421]).