Augustine was born in the year 354 in what is now Algeria. Pelagius went to Africa in 410. Augustine started writing pamphlets against Pelagius in the year 412. He died in the year 430. Pelagius may have been a Celtic monk from England or Ireland. A book by H. Zimmer, "Pelagius in Ireland", says that Pelagius was Irish.

St. Augustine complained that the Pelagians saw no urgent need to baptize infants. This conflicting view of baptism returned during the 16th century protestant reformation, at least for some factions. Pelagius believed that people have free will while Augustine believed in predestination. Pelagius thought that one who had been baptised as a Christian might still do evil things and go to hell, but that God would not punish an infant just for lack of baptism.

According to the book "Augustine", 2005, by James J. O'Donnell, Augustine and Pelagius competed for attention from the rich and powerful in Rome. Both sent letters hoping that their ideas would become more accepted. Pelagius may have been a little more successful than Augustine, at least until Pelagius was tried for heresy in Jerusalem. Pelagius was aquitted, but some of his followers were not.

According to "Europe, A History", 1996, by Norman Davies, friends of Pelagius called him "Brito". This likely indicates that he was from the British Isles. He may have been Welsh or Irish. Davies says that Pelagius contributed to Christian thought mostly by provoking Augustine to clarify various issues.

According to the wikipedia entry on Pelagius, when Alaric captured and looted Rome in 410, Pelagius fled to Carthage and met Saint Augustine.

While ancient Greeks and Celts understood that the earth is spherical (or nearly sperical), Augustine, believing that the earth was flat, wrote:

As to the fable that there are Antipodes, that is to say, men on the opposite side of the earth, where the sun rises when it sets on us, men who walk with their feet opposite ours, there is no reason for believing it. Those who affirm it do not claim to possess any actual information; they merely conjecture that, since the earth is suspended within the concavity of the heavens, and there is as much room on the one side of it as on the other, therefore the part which is beneath cannot be void of human inhabitants. They fail to notice that, even should it be believed or demonstrated that the world is round or spherical in form, it does not follow that the part of the earth opposite to us is not completely covered with water, or that any conjectured dry land there should be inhabited by men. For Scripture, which confirms the truth of its historical statements by the accomplishment of its prophecies, teaches not falsehood; and it is too absurd to say that some men might have set sail from this side and, traversing the immense expanse of ocean, have propagated there a race of human beings descended from that one first man.

According to a Catholic Encyclopedia, the main ideas that Pelagius was accused of were:

  1. Even if Adam had not sinned, he would have died.
  2. Adam's sin harmed only himself, not the human race.
  3. Children just born are in the same state as Adam before his fall.
  4. The whole human race neither dies through Adam's sin or death, nor rises again through the resurrection of Christ.
  5. The (Mosaic Law) is as good a guide to heaven as the Gospel.
  6. Even before the advent of Christ there were men who were without sin.

In The Ecclesiastical History of M. L'abbe Fleury, from A.D. 400 to A.D. 429 By Claude Fleury, page 238-239, is a quote from a letter to Demetrias. Pelagius writes: "If then men without God thus manifest how God has made them, consider what Christians may do, whose nature and life have been trained to better things, and who are even assisted by the Divine grace." Pelagius also wrote that God is "too good to condemn man for evils which he could not avoid". This was countered by Augustine with the concept of guilt inherited from Adam. The letters from Pelagius to Demetrias upset Augustine. The official story is that the theology of Pelagius did not agree with that of Augustine. However, one might note that Demetrias was fleeing from Rome and the conquest of Rome by Alaric with much of her family's wealth. Since wealth was held in common by the churches in Africa, there was much money at stake for the theologian that could persuade Demetrias to join that theologian's congregation.