Both the East and Western Churches officially solved the issues with Christ's deity. Rome, however, with their solid views of Jesus, had to decide exactly how God saved man.
Pelagius came from Britain to Rome to preach his soteriology (how folks are saved). Augustine, who struggled with a complete depravity before his conversion, completely opposed Pelagius. I think the best way to describe these views is to completely quote Earle Cairns from chapter 12, pages 137 to 138 from his book Christianity through the Centuries.
"Pelagius believed that each man is created free as Adam was and that each man has the power to choose good or evil. Each soul is a separate creation of God, and therefore, uncontaminated by the sin of Adam."
Pelagius taught that people could completely follow God with their free wills with Christ as just an example.
Augustine "opposed what he believed was a denial of the grace of God by insisting that regeneration is exclusively the work of the Holy Spirit." To clarify the syntax there, Augustine blew the whistle and said that Pelagius denied God's grace. The Bishop of Hippo, however, preached that only the Holy Spirit can save.
"Man was originally made in the image of God and free to choose good and evil, but Adam's sin bound all men because Adam was the head of the race. Man's will is entirely corrupted by the Fall so that he must be considered totally depraved and unable to exercise his will in regard to the matter of salvation. Augustine believed that all inherit sin through Adam and that no one, therefore, can escape original sin. Salvation can come only to the elect through the grace of God in Christ. God must energize the human will to accept His proffered grace, which is only for those whom He has elected to salvation."
The Council of Ephesus condemned Pelagius's views as heresy in 431. Later, John Cassian came along with a compromise between Pelagius and Augustine. "He taught that all men are sinful because of the Fall and that their wills are weakened but not totally corrupted. Man's partially free will can cooperate with divine grace in the process of salvation." This is not as wicked because in this view, men still need God's grace for salvation. However, the Synod of orange condemned this in 529 as Semi-Pelagian and went with Augustine's monergistic view.
To the credit of both Pelagius and Cassian, they did fear that people would take advantage of God's grace as a license to sin. This is a legitimate fear. However, those who have not been changed by God's grace through Christ, have not truly been saved. A life will show fruit of God's election.
This controversy has never totally been solved. "Twentieth-century liberal thought is only a resurgence of the Pelagian idea that man can achieve salvation by cooperation with the divine will through his own efforts." This ignores the point that man, descended from Adam, and all born outside of God's presence, will only choose evil with his free will until Christ intervenes and issues them the Holy Spirit. The increasing evil in the world only proves this.
So, with the last three posts, the Church developed the Niceane Creed, the Chalcedonian Creed, the Athanasian Creed, and many more to teach in simple terms these complex theologies.