Christian heresy of the 5th century that emphasized free will and the goodness of human nature. Pelagius (354?–after 418), a British monk who settled in Africa in 410, was eager to raise moral standards among Christians. Rejecting the arguments of those who attributed their sins to human weakness, he argued that God made humans free to choose between good and evil and that sin is an entirely voluntary act. His disciple Celestius denied the church's doctrine of original sin and the necessity of infant baptism. Pelagius and Celestius were excommunicated in 418, but their views continued to find defenders until the Council of Ephesus condemned Pelagianism in 431.