Pelagius believed in justification by faith alone. This was heresy. To see why this as the case, a discussion of baptism is in order. It was generally agreed that infants should be baptized; however, there was dispute as to what this implied about their spiritual condition. The most common position was that baptism was necessary to wash away original sin. Pelagius denied this. From a Reformed point of view he was right, but he was still a heretic. The heresy was due to why he was right. Pelegius did not believe that baptism washed away original sin because he did not believe the doctrine of original sin. He used a somehwat creative hermeneutic to explain why so many people sin if they aren't born sinners. Romans 5:19 states, "For as by the one man's disobedience the many were made sinners, so by the one man's obedience the many will be made righteous." Pelagius states, "Just as by the example of Adam's disobedience many sinned, so also many are justified by Christ's obedience" (Commentary on St. Paul's Epistle to the Romans). By this, he meant the example of Christ's obedience. He gos on to say, "Great, therefore, is th ecrime of disobedience that kills so many." In other words, because Christ obeyed, we should know better than to disobey. The further implication is that it is possible to not sin. Here is his explanation of Romans 5:12:
By example or by pattern. Just as through Adam sin came at a time when it did not yet exist, so in the same way through Christ righteousness was recovered at a time when it survived in almost no one. And jsut as through the former's sin death came in, so also through the latter's righteousness life was regained. As long as they sin the same way, they likewise die. For death did not pass on to Abraham and Isaac [and Jacob], [concerning whom the Lord says: 'Truly they are all living' [Luke 20:38]. But here he says all are dead because in a multitude of sinners no exception is made for a few righteous. So also elsewhere: 'There is not one who does good, not even one' (Ps. 13:1; cf. Rom. 3:12), [and 'every] one a liar' (Rom. 3:4). Or: Death passed on all who lived in a human, [and] not a heavenly, fashion."
So how does this make justification by faith alone a heresy? Well, for one thing, it was far from the Reformed doctrine of the same name. In that doctrine, a forensic righteousness is taught. God declares his elect to be righteous. Good works are not needed before this pronouncement can be made. "Faith alone" is then understood to mean faith apart from works. However, in Pelagius' view, man, by his own merit, is able to save himself without assistance from God. He simply needs to have faith in the obedience of Christ and then copy it. "Faith alone" is then understood to mean faith apart from grace. Pelagius denied that this was exactly what he meant.
Because of his talent for prevarication, a series of synods ensued before he was finally declared a heretic. The first, at Carthage in 411, was actually against Caelestius; however, he held to the main points of Pelagianism. These were as follows:
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 the fall.
4. The whole human race neither dies through Adam's sin or death, nor rised 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.
He was excommunicated and appealed to Rome.
The next event happened when Augustine sent the Spanish priest Orosius to Palestine to warn Jerome about Pelagianism. A synod was convened in Bethlehem in 415 presided over by Bishop John of Jerusalem. it was decided that since John did not speak the language of either of the parties, the Latins could best handle it. A letter to this effect was sent to Pope Innocent; however, before anything could happen, another synod was convened that same year in Diospolis, Palestine. Pelagius' accusers were unable to show up due to illness. Parts of their letter of accusation was read in mistranslation to the bishops who only spoke Greek. Fortunately for Pelagius, he also could speak Greek. He managed to convince the bishops that his true beliefs weren't anything like they had been made to sound. The synod at Carthage was told. They confirmed the pronouncement of 411 against Pelagianism. In 416, the synod at Mileve, at which Augustine was present, joined them in sending an appeal to the pope. He confirmed the decision at Carthage and Pelagius was excommunicated. Then the pope died.
When the new pope came into office, Pelagius managed to convince him that he had been wronged. In 418, the bishops at Carthage, hearing this, sent as letter to the pope asking "that he should uphold the sentence pronounced by Pope Innocent against Pelagius and Caelestus, until both of them distinctly acknowledged that for every single good action we need the help of the grace of God through Jesus Christ; and this not only to perceive what is right, but also to practice it, so that without it we can neither possess, think, speak, or do anything really good and holy" (from Hefele, A History of the Councils of the Church). Evidently, the pope didn't want to deal with it. Instead, he sent them all the appropriate documents for common consultation. They used this opportunity for a General Synod at Carthage, which finally anathematized Pelagianism.
Pelagius, up to this point, had been able to lie his way out of being convicted. Perhaps Augustine, who had tteh best grasp of the true doctrine, deserves the most credit for stopping this. In reference to Pelagius' denial of the necessity of grace for justification, he says, "The bishops believed that Pelagius confessed this grace, which they knew was commonly accepted in the catholic Church." However, Augustin goes on to say that Pelagius, prior to this had "very explicitly admitted that he understood by the 'grace of God' that, when our nature was created, it received the posibility of not sinning, because it was created with a free will. -And thus while the bishops understood him to mean by 'grace,' not that by which we humans were created, but that by which we have been made new creatures by adoption (since it is this latter grace which divine Scripture very clearly commends), they, not realizing he was a heretic, acquitted him as a catholic."