Pelagius was a British born monk who settled in Rome around 380 and became known there as a serious moral teacher rather than a speculative theologian; his early writings are largely practical guidance on Christian conduct. His central claim was that God, in creating human beings with free will, gave them the genuine and lasting capacity to choose good without sin, a capacity that Adam's disobedience did not remove from his descendants. On this view sin is learned through habit and bad example rather than inherited as a corrupted nature, and moral failure is a matter of the will choosing wrongly, not of a condition the will cannot overcome unaided.
Pelagius framed this as a call to responsibility: if human nature is not the culprit, believers cannot excuse their sins by blaming an inherited weakness, and must instead exert the effort God's commandments assume they are capable of. His student Caelestius pressed the position further and more provocatively than Pelagius himself is recorded as doing, arguing that Adam's sin harmed only Adam, that infants are born in the same state Adam occupied before the fall, and that infant death without baptism carries no penalty, claims that drew the earliest formal condemnation, at the Council of Carthage in 411, before Pelagius was targeted by name.
Augustine of Hippo became the chief opponent once the debate reached North Africa, arguing from Paul's Letter to the Romans and from his own account of a will too weak to reform itself without grace, recorded in his Confessions, that human nature after Adam is incapable of turning to good without a prior, freely given act of divine grace.