Question: "Is the teaching that ultimately all men will be saved (the apokatastasis) a heresy, or is it an acceptable theological opinion within the bounds of Orthodoxy?"
Origen taught the heretical doctrine of the apokatastasis, that ultimately everyone, even the devil, would be saved. The Church condemned this teaching at the Fifth Ecumenical Council. The Church has ever taught this as a fact since that time. However, in recent times we have had a rebirth of this heresy, and have many who try to argue that the Fifth Ecumenical Council did not condemn this teaching.
Did the Fifth Ecumenical Council Anathematize this Heresy?
To cite some examples of trustworthy theologians who state this in no uncertain terms, Fr. Michael Pomazansky wrote:
"The Church, basing itself on the word of God, acknowledges the torments of gehenna to be eternal and unending, and therefore it condemned at the Fifth Ecumenical Council the false teaching of the Origenists that the demons and impious people would suffer in hell only for a certain definite time, and then would be restored to their original condition of innocence (apokatastasis in Greek). The condemnation at the Universal Judgment is called in the Apocalypse of St. John the Theologian the "second death" (Apoc. 20:14).
An attempt to understand the torments of gehenna in a relative sense, to understand eternity as some kind of age or period — perhaps a long one, but one still having an end — was made in antiquity, just as it is made today; this view in general denies the reality of these torments. In this attempt there are brought forward conceptions of a logical kind: the disharmony between such torments and the goodness of God is pointed out, as is the seeming disproportion between crimes that are temporal and the eternity of the punishments for sin, as well as the disharmony between these eternal punishments and the final aim of the creation of man, which is blessedness in God.
But it is not for us to define the boundaries between the unutterable mercy of God and His justice or righteousness. We know that the Lord "will have all men to be saved, and to come unto the knowledge of the truth" (1 Tim. 2:4); but man is capable, through his own evil will, of rejecting the mercy of God and the means of salvation. Chrysostom, in interpreting the depiction of the Last Judgment, remarks: "When He (the Lord) spoke about the Kingdom, after saying, ‘Come, ye blessed of My Father, inherit the Kingdom,’ He added, ‘which is prepared for you from the foundation of the world’ (Matt. 25:34), but when speaking about the fire, He did not speak thus, but He added: which is ‘prepared for the devil and his angels’ (Matt. 25:41). For I have prepared for you a Kingdom, but the fire I have prepared not for you but for the devil and his angels. But since you have cast your own selves into the fire, therefore accuse yourself for this" (Homily 70 on Matthew).
We have no right to understand the words of the Lord only conditionally, as a threat or as a certain pedagogical means applied by the Saviour. If we understand it this way we err, since the Saviour does not instill in us any such understanding, and we subject ourselves to God’s wrath according to the word of the Psalmist: "Why hath the ungodly one provoked God? For he hath said in his heart: He will not make enquiry" (Ps. 9:34) (Orthodox Dogmatic Theology (Platina, CA: St. Herman Press, 1984, p. 349f).
Metropolitan Hierotheos (Vlachos) devotes an entire chapter to this subject in his book "Life After Death (Chapter 8 The restoration of all things, pp. 273-312), affirms that this heresy was condemned by the Fifth Ecumenical Council, and goes to great lengths to make the case that St. Gregory of Nyssa did not in fact teach it, but rather taught that hell (gehenna) and its punishments are unending, and that those who attribute this teaching to him are simply failing to understand them in the context of his complete teachings on the subject. If one rejects the argument that St. Gregory of Nyssa did not teach this doctrine, that would only prove St. Gregory to be in error, because Ecumenical Councils are infallible, whereas no Church Father, as an individual, is. However, it certainly is interesting that in the one instance in which, if he was a universalist, you would expect him to put that on display, St. Gregory of Nyssa not only does not affirm universalism in his treatise on the death of unbaptized infants, but directly refutes it when speaking of Judas as an example of one who died in his sins:
"Certainly, in comparison with one who has lived all his life in sin, not only the innocent babe but even one who has never come into the world at all will be blessed. We learn as much too in the case of Judas, from the sentence pronounced upon him in the Gospels; namely, that when we think of such men, that which never existed is to be preferred to that which has existed in such sin. For, as to the latter, on account of the depth of the ingrained evil, the chastisement in the way of purgation will be extended into infinity..." (On Infants' Early Deaths).
Anathemas? What Anathemas?
The advocates of Universalism try to argue that, despite the fact that the Church has consistently stated that the Fifth Ecumenical Council anathematized this heresy, that there are reasons to doubt whether the council formally issued the anathemas ascribed to it.
St. Justinian issued his anathemas against Origen before the Council, which he convoked, and the last of those anathemas is as follows:
"If anyone says or thinks that the punishment of demons and of impious men is only temporary, and will one day have an end, and that a restoration (ἀποκατάστασις) will take place of demons and of impious men, let him be anathema."
The first of the Council's anathemas states:
"If anyone asserts the fabulous pre-existence of souls, and shall assert the monstrous restoration (ἀποκατάστασις) which follows from it: let him be anathema."
Now, for the sake of argument, let's assume that there is some ambiguity about whether or not these anathemas were endorsed by that council. All one has to do to settle the question is to consider the Synodikon of Orthodoxy which is recited every year, throughout the Orthodox Church, on the Sunday of Orthodox (the first Sunday of Lent):
"To them who accept and transmit the vain Greek teachings that there is a pre-existence of souls and teach that all things were not produced and did not come into existence out of non-being, that there is an end to the torment or a restoration again of creation and of human affairs, meaning by such teachings that the Kingdom of Heaven is entirely perishable and fleeting, whereas the Kingdom of Heaven is eternal and indissoluble as Christ our God Himself taught and delivered to us, and as we have ascertained from the entire Old and New Testaments, that the torment is unending and the Kingdom everlasting, to them who by such teachings both destroy themselves and become agents of eternal condemnation to others, Anathema! Anathema! Anathema!"
Those who advocate for universalism argue that this is only a condemnation of Origen's universalism, not the universalism supposedly expressed by other Fathers, because they had different theological and philosophical reasons for their universalism. But that is a bit like arguing that the Church hasn't anathematized Jehovah's Witness Christology, because they have different theological reasons for denying the divinity of Christ than the Arians did. This anathema states, without equivocation, that "we have ascertained from the entire Old and New Testaments, that the torment is unending and the Kingdom everlasting..." and there is no indication that we would ascertain anything differently if people were universalists because they saw a documentary on the history channel, read pseudo-Isaac's writings, and agreed with it, or agreed with Origen.
Anyone who has ever had an Orthodox thought in their life knows that we believe what we say in the services of the Church (lex orandi lex credendi), and when what we say ends with "Anathema!", we mean it in no uncertain terms.
What Saith the Scriptures?
If one believes Christ's teachings carry any weight, He affirms the unending character of the torments of hell repeatedly:
In Mark Chapter Nine, he states that the fires of hell (gehenna) will not be quenched five times, and speaks of the worm that will not die three times:
"And if thy hand offend thee, cut it off: it is better for thee to enter into life maimed, than having two hands to go into hell [gehenna], into the fire that never shall be quenched: where their worm dieth not, and the fire is not quenched. And if thy foot offend thee, cut it off: it is better for thee to enter halt into life, than having two feet to be cast into hell, into the fire that never shall be quenched: Where their worm dieth not, and the fire is not quenched. And if thine eye offend thee, pluck it out: it is better for thee to enter into the kingdom of God with one eye, than having two eyes to be cast into hell fire: where their worm dieth not, and the fire is not quenched (Mark 9:43-48).
When Christ speaks of gehenna in these terms, he is probably alluding to Isaiah 66:24: "And they shall go forth, and look upon the carcasses of the men that have transgressed against me: for their worm shall not die, neither shall their fire be quenched; and they shall be an abhorring unto all flesh"; and Judith 16:17: "Woe to the nations that rise up against my people! The Lord Almighty will take vengeance on them in the day of judgment; he will send fire and worms into their flesh; they shall weep in pain forever."
In the parable of the Sheep and the Goats, Christ addresses the wicked (the goats) and said: "Depart from me, ye cursed, into everlasting fire, prepared for the devil and his angels" (Matthew 25:26); and he concludes the parable by saying: "And these shall go away into everlasting punishment: but the righteous into life eternal (Matthew 25:46).
St. Paul wrote: "since it is a righteous thing with God to repay with tribulation those who trouble you, and to give you who are troubled rest with us when the Lord Jesus is revealed from heaven with His mighty angels, in flaming fire taking vengeance on those who do not know God, and on those who do not obey the gospel of our Lord Jesus Christ. These shall be punished with everlasting destruction from the presence of the Lord and from the glory of His power" (2 Thessalonians 1:6-9).
Commenting on these verses, St. John Chrysostom wrote:
"There are many men, who form good hopes not by abstaining from their sins, but by thinking that hell is not so terrible as it is said to be, but milder than what is threatened, and temporary, not eternal; and about this they philosophize much. But I could show from many reasons, and conclude from the very expressions concerning hell, that it is not only not milder, but much more terrible than is threatened. But I do not now intend to discourse concerning these things. For the fear even from bare words is sufficient, though we do not fully unfold their meaning. But that it is not temporary, hear Paul now saying, concerning those who know not God, and who do not believe in the Gospel, that “they shall suffer punishment, even eternal destruction.” How then is that temporary which is everlasting? “From the face of the Lord,” he says. What is this? He here wishes to say how easily it might be. For since they were then much puffed up, there is no need, he says, of much trouble; it is enough that God comes and is seen, and all are involved in punishment and vengeance. His coming only to some indeed will be Light, but to others vengeance" (Homily 3, 2nd Thessalonians).
Update: Here is an interesting comment from St. Cyril of Alexandria, on 1 Peter 3:19:
"Here Peter answers the question which some objectors have raised, namely, if the incarnation was so beneficial, why was Christ not incarnated for such a long time, given that he went to the spirits which were in prison and preached to them also? In order to deliver all those who would believe, Christ taught those who were alive on earth at the time of his incarnation, and these others acknowledged him when he appeared to them in the lower regions, and thus they too benefited from his coming. Going in his soul, he preached to those who were in hell, appearing to them as one soul to other souls. When the gatekeepers of hell saw him, they fled; the bronze gates were broken open, and the iron chains were undone. And the only-begotten Son shouted with authority to the suffering souls, according to the word of the new covenant, saying to those in chains: "Come out!" and to those in darkness: "Be enlightened." In other words, he preached to those who were in hell also, so that he might save all those who would believe in him. For both those who were alive on earth during the time of his incarnation and those who were in hell had a chance to acknowledge him. The greater part of the new covenant is beyond nature and tradition, so that while Christ was able to preach to all those who were alive at the time of his appearing and those who believed in him were blessed, so too he was able to liberate those in hell who believed and acknowledged him, by his descent there. However, the souls of those who practiced idolatry and outrageous ungodliness, as well as those who were blinded by fleshly lusts, did not have the power to see him, and they were not delivered." (Ancient Christian Commentary on Scripture, New Testament, Vol. XI, James, 1-2 Peter, 1-3 John, Jude, Gerald Bray, ed. (Downers Grove, IL: Intervasity Press, 2000) p. 107f).