A British monk read a prayer in Augustine's Confessions and was appalled. "Give what you command, and command what you will." To Pelagius it sounded like an excuse for moral laziness. To Augustine it was the whole gospel. The argument that followed decided how the Western church talks about sin and grace to this day.
That Pelagius taught people to save themselves by good works, and Augustine taught grace. That is roughly the outcome, but it makes Pelagius a cartoon. He talked about grace constantly. The fight was over what grace is.
That Augustine invented original sin. The idea that Adam's fall affects the whole human race is older than Augustine. What he did was press it further than anyone had, and make it the centre of the argument.
That it was an abstract argument about free will. It began as a pastoral one: how should Christians in Rome, many of them comfortable and half-hearted, be called to live?
The question was what a human being can do before God helps them. Pelagius held that God would never command what we are unable to do. So our nature, as God made it, is able to choose good. Adam harmed us by setting a bad example, not by passing on a damaged nature. Grace is the gift of that nature, of the law, of Christ's teaching and example, and of forgiveness for past sins.
Augustine held that since Adam, the human will is bent. We are not neutral people choosing between good and evil; we love the wrong things, and cannot by ourselves start loving the right ones. So grace is not only teaching and example. God must change the will itself, and he must act first. Even the beginning of faith is his gift.
Both men were serious, ascetic Christians. Both were trying to protect something. Pelagius wanted to protect human responsibility, and the justice of a God who commands. Augustine wanted to protect the truth that salvation is God's gift from start to finish, so that no one can boast.
Each stated as its holders would have stated it. The third position is the one usually left out.
Pelagius, and more sharply his follower Caelestius
God commands nothing impossible. Human nature can choose the good. Grace teaches, shows the way in Christ, and forgives, but the choosing is ours.
Why they held it Because a God who commanded what we cannot do would be unjust, and because teaching people they cannot obey gives them an excuse not to try.
Augustine of Hippo, and the bishops of North Africa
Since the fall, every person inherits a will turned away from God. Grace must go before, change the will, and keep it. Even faith is God's gift.
Why they held it Because Scripture says we are dead in sin and that faith is a gift (Ephesians 2:1–9), and because the church baptised infants for the forgiveness of sins, which makes no sense if they had none.
John Cassian and monks in southern Gaul (later called "Semi-Pelagians", a sixteenth-century label)
Grace is necessary, but the first movement toward God can come from the human side, which God then meets and helps.
Why they held it Because Augustine's strongest statements seemed to make human effort pointless, and monastic life is built on effort.
Pelagius was acquitted once and condemned several times. The middle position took another century to resolve.
The argument did not end there. It came back in the Reformation, between Calvinists and Arminians, and between Jansenists and Jesuits, and it is still live wherever Christians discuss what part, if any, the human will plays in coming to faith.
Because almost every Christian prays like an Augustinian ("Lord, change my heart") and many preach like Pelagius ("try harder"). Noticing which one you are doing, and why, is most of what this argument has to teach.