Augustine heard of Pelagius’ heretic doctrines from others, but wanted to hear it from Pelagius himself before accusing him so that he could not deny it. In his On the Proceedings of Pelagius, Augustine writes,
I first became acquainted with Pelagius’ name, along with praise for him, at a distance, and when he was living at Rome. Afterwards reports began to reach us, that he disputed against the grace of God. This caused me much pain, for I could not refuse to believe the statements of my informants; but yet I was desirous of ascertaining information on the matter either from himself or from some treatise of his, that, in case I should have to discuss the question with him, it should be on grounds he could not disown.
Augustine began his campaign against Pelagianism with sermons that were directed against the errors taught by Pelagius and his supporters. He had not received what he had hoped for pertaining to the heresy from Pelagius or from his writings. Therefore, Augustine decided to proceed with sermons against Pelagius’ doctrines. In these sermons he made the decision not to use names of the perpetrators, but instead he preached against the errors themselves. He did this in the hopes that it would allow the perpetrators to return to the right doctrines of Christianity instead of being punished for their errors.
The first real book that Augustine wrote against Pelagianism was titled On the Merits and Remission of Sins and On the Baptism of Infants; they were written in 412. Augustine wrote these books because Marcellinus, an official presiding over the “conference of the catholics and Donatists,” wrote to Augustine asking questions pertaining to Pelagian doctrine. The questions were related to the connection of death with sin, the transmission of sin, living a sinless life, and infants needing baptism. The answers to these questions filled the pages of two volumes.
After Augustine had completed these two books he came into possession of a book by Pelagius titled Commentary on Paul’s Epistles. He realized that there were things within those pages that he had not addressed in the two books that he had written for Marcellinus, so Augustine wrote a third volume wherein he says, “Now I confess that I have not refuted this argument in my lengthy treatise, because it did not indeed once occur to me that anybody was capable of thinking such sentiments.” Augustine could not believe anyone could write such things as Pelagius had written in his commentary.
From these books stemmed more questions from Marcellinus that concerned the assertion of man’s ability to live a sinless life. This led to the book titled On the Spirit and the Letter; written in 412 as well. Within the pages of this book Augustine writes on the absolute necessity of God’s grace to live a good life; he argues for original sin, imperfection of man’s righteousness, and the necessity of grace.
The controversy returned to private discourses and sermons for about three years after these books had been written. During that time in 414, Augustine gives us a glimpse into the advancement of Pelagianism in a letter that he had written. The Pelagians were spreading their error everywhere, and in 414 Augustine received questions written by Hilary from Sicily about strange doctrines being spread in Syracuse by certain Christians.
These strange doctrines were Pelagian doctrines that were being spread by Pelagius’ followers. Augustine answered Hilary in a letter, wherein he revisits the arguments that he had put forth in two of his previous books to Marcellinus; On the Merits and Forgiveness of Sins, and On the Spirit and the Letter.
Two young men who had repented from their error of following the Pelagian doctrines wrote to Augustine, and along with the letter they sent a copy of a book written by Pelagius. In the letter they asked Augustine to answer what was written in Pelagius’ book to exhort Pelagius to repent. The result is the book titled On Nature and Grace; written in 415. Up until this time Augustine had refrained from calling Pelagius a heretic, but that changed after he had read this book. Thus, his attitude towards Pelagius was becoming strained by the error of Pelagius’ doctrines.
Coming out of Sicily in 415, two exiled Spanish Bishops brought out of that country a paper titled Definitions Ascribed to Coelestius, which confirmed Augustine’s assumption that the trouble started in Sicily was grounded in Coelestius. In this paper the Pelagians were trying to logically force the catholics to “admit that man can live in this world without sin.” Augustine answered with his book titled On the Perfection of Man’s Righteousness; written in 415. In these pages he points out that man can live without sin if God wills it, such is what Coelestius writes, but he continues if God gives him His help. This is what he says Coelestius so conveniently leaves out of his argument; that God will give him His help.
At the request of two friends, Pinianus and Melania, husband and wife, Augustine wrote two books against Pelagius titled On the Grace of Christ, and On Original Sin; written in 418. These two friends of Augustine had come to know Pelagius and were being influenced by some of his teachings, so they wanted Augustine to write an answer to what they had learned from Pelagius concerning grace and infant baptism. In the first book, Augustine addressed the doctrine of grace, and in the second infant baptism. In On the Grace of Christ Augustine argues for the necessity of God’s grace to help us towards our justification and that He orchestrates all things to bring out good in our lives. While in the second book, On Original Sin, he argues that sin entered into the world by one man and death entered by sin and that sin has been passed on to all men.
In 419, the Pelagians accused Augustine of condemning marriage because of his defense of original sin. The Pelagians reasoned that if man is born with sin and marriage results in children, then marriage itself is sinful. Augustine answered this accusation with his book titled On Marriage and Concupiscence, wherein Augustine argues that marriage is good and God is the maker of the children that come forth from the marriage.
The Pelagians argued that if the soul was created new at the time of birth, then it would not be fair of God to put Adam’s sin to rest on the soul. To this argument Augustine had a hard time finding an answer and turned to Jerome for an answer. Jerome affirmed the belief that the soul was made new at the time of birth, so it seemed to Augustine that Jerome would be the natural source to seek an answer to this argument presented to the Pelagians. Jerome replied with a letter stating that he was too busy to answer this argument. At the end of 419 Augustine presented an answer in his book titled On the Soul and its Origin.
The Pelagians themselves were busy writing rebuttals to the writings of Augustine, and Augustine wanted to read the complete copies of the letters before writing a defense against them. His answer to these letters from his opponents consists of four volumes titled Against Two Letters of the Pelagians; written in 420.
There was a pause in the back and forth arguing between Augustine and the Pelagians for a period of about five years, at which time a monk came across some of Augustine’s writings and read them out loud to some of the monks in his monastery around 426. The writings caused an outrage among some of the monks who were untrained theologically. They argued that free will was destroyed as a result of Augustine’s arguments found in his writings. This led to Augustine writing his book titled On Grace and Free Will. He argued within these pages that free will and grace must be proclaimed together.
This led to another book titled On Rebuke and Grace, wherein Augustine writes on the relationship of God’s grace to our conduct as humans. Some in the monastery had argued that man could not be held liable for being unable to do good and should seek for God to do the good for him.
By this time pure Pelagianism had faded out and semi-Pelagianism took its place. The essential doctrine of semi-Pelagianism is that faith does not begin with God but with the act of man’s free will. In answer to semi-Pelagianism, Augustine wrote two last books titled On the Predestination of the Saints and The Gift of Perseverence; both written in 428-429.
In between these writings and arguments Augustine wrote many letters to those who were in search of answers against the Pelagian doctrines that seemed strange to them. Augustine spent considerable effort and time refuting the heretics of Pelagianism, but out of all of this the one main point that Augustine was arguing was that of his theology of grace.
Aurelius Augustine. “A Work on the Proceedings of Pelagius.” In Nicene And Post-Nicene Fathers, Volume 5: Augustine: Anti-Pelagian Writings, edited by Philip Schaff, 183-212. Peabody, MA: Hendrickson Publishers, Inc., 2004, 203.
 Ibid., 203.
 B.B. Warfield. “Introductory Essay On Augustine And The Pelagian Controversy.” In Nicene And Post-Nicene Fathers, Volume 5: Augustine: Anti-Pelagian Writings, edited by Philip Schaff, xiii-lxxi. Peabody, MA: Hendrickson Publishers, Inc., 2004, xxiv.
Aurelius Augustine. “On Forgiveness of Sins and Baptism.” In Nicene And Post-Nicene Fathers, Volume 5: Augustine: Anti-Pelagian Writings, edited by Philip Schaff, 15-78. Peabody, MA: Hendrickson Publishers, Inc., 2004, 69.
 Ibid., xxxi.
 Ibid., xlii.
 Ibid., li.
 Ibid., lix.