In St. Augustine’s earlier writings, the ideas of predestination, election, and God’s total sovereignty reign supreme. At the same time, though, he wrote once to Jerome, "But when we come to the punishment of little children, I am bewildered by great perplexities, and I am quite at a loss for an answer. I refer not only to the punishment which results from the condemnation which inevitably falls on them if they depart from the body without the sacrament of Christian grace [baptism], but the suffering which takes place before our eyes in this life and brings us sorrow....” How can a loving God cast a newborn baby into hell?
Both the Western Pope and the Eastern Patriarch found great problems in Augustine’s writings. He was certainly a great orator and author, but perhaps his oratorical skill was getting out of his spiritual depth. Enter St. John Cassian, who was asked by both Pope and Patriarch to carefully refute the extreme positions of Augustine along with the errors of Pelagianism and Nestorianism. In Augustine’s introduction to his last and perhaps greatest work, The City of God, he carefully climbs down from his high horse and accepts a mild form of “synergy,” as Cassian propounded.
This is only a brief summary of my full explanation of the intrigues between all these players in both East and West. for the full story, go to the sections on Augustine and Cassian on my literature page: https://agape-biblia.org/literatura/#Augustine