Q. 1. What is the heresy of "Pelagianism?"
A. 1. The heresy of "Pelagianism" was a belief that original sin did not taint the human nature and that the mortal will was still capable of choosing good or evil without Divine aid. In summary, it was the rejection of original sin and the Catholic practise of infant baptism.
Pelagianism was named after Pelagius (ad. 354 – ad. 420/440). His beliefs were later developed by Celestius and Julian of Eclanum into a complete system. It was refuted by Augustine of Hippo (who had for a time (385-395) held similar opinions).
The heresy of Pelagianism was attacked in the Council of Diospolis and condemned in 418 A.D. at the Council of Carthage. The decision was confirmed at the Council of Ephesus in 431 A.D.