One small word, grace, split Catholics from Protestants and Calvinists from Arminians
Christians agree that grace is a gift from God nobody can earn. The fights begin over how it reaches people and whether humans can refuse it. That question has been called the watershed dividing Catholic from Protestant, Calvinist from Arminian, and liberal theology from conservative.
The English word renders Greek charis, meaning whatever brings delight, joy or good fortune. The Septuagint used it for a Hebrew term in Genesis explaining why Noah was spared the flood. In Western theology grace is God's favour and a share in divine life, a free and unexpected gift, with the first move always made by God. Eastern Orthodoxy frames it differently, as the uncreated energies of God and a participation in divine nature, never a created substance to be handled like a commodity.
The oldest great quarrel pitted Augustine of Hippo against Pelagius, an ascetic said to have come from Britain, who saw moral slackness in early fifth-century Rome and blamed teachings that downplayed free will. Augustine answered largely from the overstated claims of Pelagius's followers, and the Council of Carthage condemned Pelagianism in 418. Pelagius himself probably held what later came to be called semi-Pelagianism, a view in which humans cooperate with grace; John Cassian taught something similar, and Eastern Orthodoxy holds it still.
The Reformation reopened everything. Calvinists stress human helplessness without grace and teach that grace is irresistible, granted to those predestined for salvation. Arminians see it working together with free will. Lutherans locate grace in Word and sacrament, and John Wesley called the Eucharist its grand channel. The Council of Trent replied in 1547 that justification comes through Christ's merit alone, yet the will, weakened by Adam's fall but not destroyed, can consent to grace or resist it.
Catholicism had its own version of the Calvinist debate. Cornelius Jansen's 1640 book Augustinus pressed hard on depravity and predestination, and his followers gathered around the Port-Royal convents, with Blaise Pascal mocking what he saw as Jesuit laxity. Jansenism was condemned as heresy for resembling Calvinism. Centuries later, a joint Catholic and Lutheran declaration affirmed that salvation depends completely on God's grace.
Source: Grace in Christianity
