Thats not how you say...the Christianity
Oh.the Islam
the Islam
the Islam
The atheist thought
the Islam
the Islam
the Islam
I do apologise, normally I do not pick out grammar issues, but you have made the same mistake enough times (or your autocorrect has picked it up as a habit of your writing) that I thought I should mention the concept of proper names.
In short, they behave like nouns but in the singular to specific things which are conceptually referring to a singular entity/idea (usually).
The reason this is important is context. "Hitler" for instance is a proper name. "Hitler did terrible things!" As a sentence indicates that the specific historical person (Adolf Hitler) did something. "The Hitler did terrible things!" Is instead giving a moniker of "Hitler" as a title, and would actually suggest you were talking about anybody BUT Adolf Hitler.
In the same context, Islam, Christianity and Atheist (the latter only when referring to it as a people/concept) are also proper names.
So when you say that the Byzantines would adopt "the Islam", it reads like "Islam" is an entity whilst removing the "the" solves the issue.
I really don't want to distract to heavily, but I felt this did need to be addressed.