First and foremost, let me thank @T.E.D for stepping in and saving us from an unprodcutive, unenjoyable edit war. I realizing moderation is supposed to be a thankless job, but I think in this case we can make an exception. Also, thank you to @SteveBird for moving the fracas to meta. Wish I had done that.
Second, let me recommend that we update the tag wiki to resolve the question. Although I would suggest that the tag wiki indicate that the tag should only be used for questions about the Protestant church beginning at the Protestant reformation. OP would probably argue that the tag should be applied to questions about the Protestant Church and the events that enabled the church. I believe that an answer is more important than the answer; that a bad concurrence is better than a virtuous argument. (Perhaps we could create two tags - one that refers to the Protestant church after the reformation and a second tag the refers to the origin of the Protestant church?
IF OP were asking about the Protestant church, I would agree that the tag would be appropriate. However, his insistence that he is looking for Protest sources prior to the Protestant reformation undermines his assertion. The tag war is a symptom.
OP is extending from Foxes assertion that all of these sects were martyrs to the Protestant faith to reach the conclusion that the Protestant Church existed before the reformation.
So let's do the comparative advantage analysis :
If we use the tag, then users searching for Protestant Church will find this question and learn things which are false; this diminishes the value of H:SE. OP's experience will not be diminished; I will assert that there is nobody who would answer the question with the tag who would not answer the question without the tag.
If we don't use the tag, OP's experience will not be changed. (He may experience an affront to his ego, but I think that is quite unavoidable at this point). Nobody's search will be diminished, since nobody is going to search for Protestant Church.
(OP would counter argue that these early dissenters were formative for the Protestant Church. I can't remember the name of that fallacy, but essentially it is a recursion problem without a stopping condition. Everything is affected by things that come before, but by that logic the big bang is formative for teh Protestant church. Common sense should apply, and we should not (in the general case) apply a tag to events the preceeded the existence of that tag.)
At least as far as I can see the greater benefit is to remove the tag.