Ironically, there is a sense in which Calvinism is Semi-Pelagian. As I write in Return to Rome:
My study of the Fathers led me to re-examine the Canons of the Council of Orange (AD 529), which, with papal sanction, rejected as heretical Pelagianism and semi-Pelagianism. Having its origin in the Catholic monk Pelagius (ca. 354–ca. 420/440), the first heresy affirms that human beings do not inherit Adam’s sin (and thus denies the doctrine of original sin) and by their free will may achieve salvation without God’s grace. On the other hand, semi-Pelagianism maintains that a human being, though weakened by original sin, may make the initial act of will toward achieving salvation prior to receiving the necessary assistance of God’s grace. The Council of Orange, in contrast, argued that Adam’s original sin is inherited by his progeny and can only be removed by the sacrament of Baptism. By the means of Baptism God’s unmerited grace is infused for the remission of sins. Then the Christian’s sanctification continues throughout his lifetime, entirely the work of the infusion of grace with which the Christian cooperates, for the Christian “does nothing good for which God is not responsible, so as to let him do it.” Even though Protestant thinkers sometimes portray the Council of Orange’s canons as a sort of paleo-Reformed document, it is the Reformation notion of imputed righteousness that, ironically, puts the Reformers partially in the Pelagian camp. This is because the Reformers and Pelagians agree that God’s infused grace is not necessary for justification.