The Collaborative International Dictionary of English v.0.48:

Unpope \Un*pope"\, v. t. [1st pref. un- + pope.]
   1. To divest of the character, office, or authority of a
      pope.
      [1913 Webster]

   2. To deprive of a pope. [Obs.]
      [1913 Webster]

            Rome will never so far unpope herself as to part
            with her pretended supremacy.         --Fuller.
      [1913 Webster]