Sentence:  Before the Reformation, whatever wars took place, there remained throughout Europe the underlying concept of an essential unity of Christendom, an idea that all states still formed part of a common body. Even after it, when Charles V came to an agreement with the <WSD>Protestantizing</WSD> princes of the empire in 1555, that idea and ideal remained. 
Meaning of the word marked with <WSD> tags:  To become Protestant; to convert to Protestantism.