the first major reformation of islam, I'd say happened at the
   time of the fall of constantiple. Before then, Islam and Eastern
   Christiandom had their political/land tussles but they
   more-or-less cooperated, as there was far more peacetime than
   wartime. But after, Islam tried to "start from scratch" again.
   That's when the troubles really began imo. An analogy would be
   around 1775 when Orthodox Judaism came out. Same kind of idea.