This also applies to other social categories such as ethnicity, but I would like to focus on religion to narrow the topic a bit.

What qualifies one to be a member of a certain religion is a matter of great controversy in the present day world. When looking at the history of religion, with it's many schisms, heresies, inquisitions and purists, that doesn't seem to be a new development. Whether or not someone is a "true believer" depends on who you ask. One persons faithful might be another persons heathen. One entity might even consider itself to be a non-member of a religion, while others might hold self-identification to be irrelevant and still judge him to be a member for other reasons.

How, then, do historians approach question that involve specific religion? If a question is about the history of, say, Buddhism, Islam, Christianity or Roman polytheism, then how do historians decide which peoples history to answer with?

Do historians simply use the least controversial definition available, thereby avoiding the need to defend their choice as much? Or is there a strict method with a lot of self-critique and conscious choice, that might lead to using controversial categories that are not acknowledged broadly? If the former, doesn't that risk leading to inconsistencies and oversights? If the latter, how do historians defend their theological categorizations from those who inevitably disagree?

Thanks in advance!