One who protests; -- originally applied to those who
   adhered to Luther, and protested against, or made a solemn declaration
   of dissent from, a decree of the Emperor Charles V. and the Diet of
   Spires, in 1529, against the Reformers, and appealed to a general
   council; -- now used in a popular sense to designate any Christian who
   does not belong to the Roman Catholic or the Greek Church.