An Anabaptist is any member of a radical sixteenth-century Christian sect of the Reformation. This religious group believed that the only form of baptism should be complete immersion of the body in water. One defining Anabaptist belief is that only adults (or those who can make an informed decision) can choose to be baptized, thus ruling out babies and young children.
Today's Mennonites, Amish, and Hutterites are all descended from that sixteenth-century sect.
In Voltaire's novel, Candide makes his way to Christian Holland, where he hopes to find charity but finds hardhearted people — except for one, an Anabaptist, who shows Candide kindness and generosity:
A man who had never been christened, a good Anabaptist, named James, beheld the cruel and ignominious treatment shown to one of his brethren, . . . he took [Candide] home, cleaned him, gave him bread and beer, presented him with two florins, and even wished to teach him the manufacture of Persian stuffs which they make in Holland.