The terms “Pelagianism,” “Semi-Pelagianism,” and “Arminianism” have in common that they all present a form of synergistic theology; that is, the beginning of man's salvation, in regeneration, is not accomplished by the sole and unilateral act of God, but is produced by God and man “working together,” in some sense. Each of these synergistic systems is in opposition to “Calvinism” or “Augustinianism,” which teaches that God sovereignly gives to each of his elect a new, living heart which cannot do otherwise than believe in Christ, and so be justified and eternally saved.
Pelagianism, the first and most radical of these synergistic theologies, was expounded by a fourth-century British monk named Pelagius. Pelagius taught that man's nature was not affected by Adam's fall, but that all men are still free to choose good or evil, to obey God or disobey him. Men are not guilty by nature, but only become guilty when they choose to do that which is evil; and Adam's failure did not corrupt his offspring, it just gave them a bad example, which they could choose to follow or not to follow. Augustine, the Bishop of Hippo, was Pelagius' great adversary, and he taught that man is bound in sin according to the scriptures, and that God's commands do not imply man's moral ability to obey them. Pelagianism was officially condemned by the Church in AD 431, at the Council of Ephesus.
“Semi-Pelagianism” is a Reformation-era term that came to designate a softer sort of Pelagianism that arose after the Council of Ephesus, in the sixth century. According to Semi-Pelagianism, man is not free to choose good or evil, but he is at least free to make the first move to God, to turn to him in faith, and so be given the power to choose good by God's grace. Man is not free to do good in his fallen nature, but he is at least able to believe and come to God in his own native strength. This softer variety of Pelagianism was officially condemned by the Church in 529, at the Council of Orange; however, the Reformers rightly recognized that the Roman church of the sixteenth century had become thoroughly Semi-Pelagian again.
“Arminianism” refers to the teachings of Jacobus Arminius, and the five points of the Remonstrance which he headed. According to Arminius, man is not so depraved that he cannot naturally seek God; God's election of men is based on his foreseeing the faith they would come to in time; the atonement of Christ was intended for every person on earth, but whether it will actually be applied to anyone in particular rests upon his free decision to believe or not to believe; God's grace is sufficient to enable men to believe if they so choose, but does not necessitate faith; and after a man has come to a genuine saving faith in Christ, he is still free to turn aside and fall away from grace, and so be eternally lost. The Synod of Dort, in 1618-1619, officially condemned Arminianism, and upheld the so-called five points of Calvinism; however, there are many Protestant churches and denominations today that hold to an Arminian theology. Arminianism differs from Semi-Pelagianism in the former's teaching on prevenient grace: against Semi-Pelagianism, Arminianism usually teaches that man does not have the natural ability to believe; however, God extends his prevenient grace to all men without exception, giving them all the moral ability to choose to believe or not to believe. Whether or not any man is actually saved depends entirely on whether a person chooses to improve upon this prevenient grace, and believe in God.