“The term [apokatastasis] is found in Justin Martyr and Irenaeus and developed into a doctrine of universal salvation by Origin. Origen was condemned by a synod in Constantinople…the general concept of a final apokatastasis is, however, found in Gregory of Nyssa and persists in a modified form in Byzantine theology, notably in Maximus the Confessor. It recurs in Modern Russian thinkers such as Solovyov, Bulgakov, and Berdiaev,” The Blackwell Dictionary of Eastern Christianity, 36.
As the implacable opponents of Eunomius, the Cappadocians were nevertheless dependent on Origen both for their biblical learning in the succession of his Hexapla and for their speculative thought; that becomes evident above all in the formulation of the doctrine of apokatastasis put forward by Gregory of Nyssa, which was spared the official condemnation visited upon Origen’s doctrines,” ibid. 482.
"Hell exists as a final possibility, but several of the Fathers have none the less believed that in the end all will be reconciled to God. It is heretical to say that all must be saved, for this is to deny free will; but it is legitimate to hope that all may be saved. Until the Last Day comes, we must not despair of anyone’s salvation, but must long and pray for the reconciliation of all without exception. No one must be excluded from our loving intercession. ‘What is a merciful heart?’ asked Isaac the Syrian. ‘It is a heart that burns with love for the whole of creation, for men, for the birds, for the beasts, for the demons, for all creatures’ (Mystic Treatises, edited by A. J. Wensinck, Amsterdam, 1923, p. 341). Gregory of Nyssa said that Christians may legitimately hope even for the redemption of the Devil."