What was Pelagianism, and why did St. Augustine oppose it?

Pelagianism was a 5th-century Christian heresy founded by Pelagius. It emphasized the essential goodness of human nature and the freedom of the human will, arguing that individuals are capable of choosing between good and evil without divine intervention. Consequently, Pelagians rejected the doctrine of original sin and the necessity of infant baptism.1

St. Augustine vehemently opposed Pelagianism because it contradicted his fundamental theological belief that humanity is inherently burdened by original sin and incapable of achieving righteousness through self-effort. Augustine asserted that humans are totally dependent upon the unmerited grace of God to fulfill the moral law and attain salvation.12

For Augustine, the Pelagian focus on human autonomy undermined the necessity of Christ’s sacrifice and the transformative power of divine grace. He maintained that since the fall of Adam, human freedom to choose good has been lost, making reliance on God’s grace the only path to spiritual life. This conflict became a defining debate in Christianity regarding free will and predestination.23