The Collaborative International Dictionary of English v.0.48:

Unprotestantize \Un*prot"es*tant*ize\, v. t. [1st pref. un- +
   protestantize.]
   To render other than Protestant; to cause to change from
   Protestantism to some other form of religion; to deprive of
   some Protestant feature or characteristic.
   [1913 Webster]

         The attempt to unprotestantize the Church of England.
                                                  --Froude.
   [1913 Webster]