unprotestantise means to cause to change from Protestantism to some other form of religion. Lexicurio rates it Sui generis — a strength score of 87 out of 100.
Etymology
From un- + Protestantise.
verb
- To cause to change from Protestantism to some other form of religion.“1867–82, James Anthony Froude, Short Studies on Great Subjects
the attempt to unprotestantise the Church of England”