The Early Church Fathers on Purgatory and praying for the dead.
Clement of Alexandria
The believer through discipline divests himself of his passions and passes to the mansion which is better than the former one, passes to the greatest torment, taking with him the characteristic of repentance for the faults he may have committed after baptism. He is tortured then still more, not yet attaining what he sees others have acquired. The greatest torments are assigned to the believer, for God’s righteousness is good, and His goodness righteous, and though these punishments cease in the course of the expiation and purification of each one, “yet” etc. (Patres Groeci. IX, col. 332 [A.D. 150-215]).
If a man departs this life with lighter faults, he is condemned to fire which burns away the lighter materials, and prepares the soul for the kingdom of God, where nothing defiled may enter. For if on the foundation of Christ you have built not only gold and silver and precious stones (I Cor., 3); but also wood and hay and stubble, what do you expect when the soul shall be separated from the body? Would you enter into heaven with your wood and hay and stubble and thus defile the kingdom of God; or on account of these hindrances would you remain without and receive no reward for your gold and silver and precious stones? Neither is this just. It remains then that you be committed to the fire which will burn the light materials; for our God to those who can comprehend heavenly things is called a cleansing fire. But this fire consumes not the creature, but what the creature has himself built, wood, and hay and stubble. It is manifest that the fire destroys the wood of our transgressions and then returns to us the reward of our great works. (Patres Groeci. XIII, col. 445, 448 [A.D. 185-232]).
The citizen of a prominent city, I erected this while I lived, that I might have a resting place for my body. Abercius is my name, a disciple of the chaste shepherd who feeds his sheep on the mountains and in the fields, who has great eyes surveying everywhere, who taught me the faithful writings of life. Standing by, I, Abercius, ordered this to be inscribed; truly I was in my seventy-second year. May everyone who is in accord with this and who understands it pray for Abercius (Epitaph of Abercius [A.D. 190]).
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]).
The faithful widow prays for the soul of her husband, and begs for him in the interim repose, and participation in the first resurrection, and offers prayers on the anniversary of his death (Monogamy 10 [A.D. 213]).
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 51:20 [A.D. 253]).
Cyril of Jerusalem
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 holy and most solemn sacrifice is laid out (Catechetical Lectures 23:5:9 [A.D. 350]).