Arminianism is a doctrine regarding salvation derived from the thought of Dutch theologian Jacobus Arminius (1560-1609). It is most notable where it parts ways with Calvinism, with which it differs on several key points, including free will, the idea of who is 'elect,' universal redemption, and the possibility of a saved person to fall from grace. In each instance, Arminianism tends to privilege free will. For example, while Calvinism holds that someone, once saved, can never fall, Arminianism insists that if a person loses faith, she is doomed; for Arminians, human beings are ever-free to fail.
Calvinists fought Arminianism, codifying their differences in the Synod of Dort of 1619, but the theology was revived in the eighteenth century by John Wesley, founder of Methodism—whose championing of Arminianism gave it a strong foothold in American religion. Wesley, with characteristic vividness, once wrote that "to say, "This man is an Arminian," has the same effect on many hearers, as to say, "This is a mad dog."