Protestant Prot"es*tant, a. [Cf. F. protestant.] 1. Making a protest; protesting. 2. Of or pertaining to the faith and practice of those Christians who reject the authority of the Roman Catholic Church; as, Protestant writers., Protestant Prot"es*tant, n. [F. protestant, fr. L. protestans, -antis, p. pr. of protestare. See Protest, v.] 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.