The belief in universal salvation is as old as Christianity itself. In this extensive work of scholarly literature Dr. Ilaria Ramelli goes into detail with the classical doctrine of universal restitution (apokatastasis) as it is found from the New Testament until the middle ages.
“The doctrine of apokatastasis, as is found, from the New Testament to Eriugena, in many Christian texts and Patristic authors, is a Christian doctrine and is grounded in Christ. This Christocentrical characterization is especially evident in Bardaisan, Origen, Gregory of Nyssa, Evagrius, and Eriugena. Indeed, the Christian doctrine of apokatastasis is based on the incarnation, death, and resurrection of Christ, and on God’s being the supreme Good. It is also founded upon God’s grace, which will “bestow mercy upon all,” and the divine will—which these Patristic authors saw as revealed by Scripture—“that all humans be saved and reach the knowledge of Truth.” They also considered it to be revealed in Scripture, and in particular in a prophecy by St. Paul, that in the telos, when all the powers of evil and death will be annihilated and all enemies will submit (for Origen and his followers, in a voluntary submission), “God will be all in all.” The apokatastasis doctrine is historically very far from having been produced by an isolated character, excessively influenced or even “contaminated” by Greek theories, such as Origen has been long considered to be.” (p. 817) (Ilaria Ramelli, The Christian Doctrine of Apokatastasis 2013).
“The doctrine of apokatastasis as the eventual universal salvation is an authentically Christian, or Jewish-Christian, doctrine. Before Christianity, no religion or philosophy had ever maintained it, not even Plato or mystery religions.” (p. 819)
“In fact, the main Patristic supporters of this theory, Origen and Nyssen, did support it in defence of Christian “orthodoxy,” against those which were regarded as the most dangerous heresies of their times, as I have argued: Origen supported it against “Gnosticism” and Marcionism, and Gregory against “Arianism.” (p. 823)
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See also J.W. Hansons classic on early Christian universalism from 1899: Universalism The Prevailing Doctrine Of The Christian Church During Its First Five Hundred Years