A very important point in my understanding of religion and its history

Over there at MEDIEV-L, we were discussing Islam, and someone mentioned Karen Armstrong, the prominent writer on religious issues. This clause was a side remark in a long argument:

...the question is not who armstrong thinks is a "real representative" of a religion that is not hers ...

And that impelled me to say this:

And for me that opens up another question: even if the religion is yours, do you get to say who is the real representative of that religion? You may think you are an X, and that all Xs believe such and such, and Y is the best representative that belief or practice, but somebody else somewhere in time and space has an equally strong contrary belief about what Xs say and do and believe, with plenty of evidence to back themselves up. If we are talking as historians and scholars, both persons' claims are ahistorical. What would be objectively verifiable is that there are certain tendencies and disagreements within religion X, and that any definition of religion X includes and excludes people who may or may not think of themselves as X.

Reader, if you care about what I have learned in decades of historical studies, this is one of the most important things.