A leading Anabaptist theologian. Simons was born in Friesland and ordained as a Roman Catholic priest, but immediately after his ordination he began doubting Roman Catholic teaching on transubstantiation, and his views of baptism and church authority underwent similar transformations over the next sixteen years. A violent uprising in Munster (1534-1535) prompted him in 1536 to promote vigorously his Anabaptist convictions, especially pacifism, in order to prevent similar catastrophes. He wrote several devotional and theological works as an itinerant minister and hunted heretics, persuading many toward pacifistic Anabaptism, particularly in North Germany and Holland. In a letter to Martin Micron, Calvin opposed Simons’s view that Jesus was only born in rather than of Mary and defended a Christology that takes seriously Jesus’ full humanity. After Simons’s death, many of his followers came to be called Mennonites.