The Collaborative International Dictionary of English v.0.48:

Mennonist \Men"non*ist\, Mennonite \Men"non*ite\, n. (Eccl.
   Hist.)
   One of a small denomination of Christians, so called from
   Menno Simons of Friesland, their founder. They believe that
   the New Testament is the only rule of faith, that there is no
   original sin, that infants should not be baptized, and that
   Christians ought not to take oath, hold office, or render
   military service.
   [1913 Webster] menobranch

WordNet (r) 3.0 (2006):

Mennonite
    n 1: a member of an Anabaptist movement in Holland noted for its
         simplicity of life