I'm genuinely interested in how this happened. I've been playing this game for a very very long time, and never seen a feudal Empire turn Theocracy before. They own a substantial amount of land as well, well.. all of the byzantine empire really.
It's a known bug. The Ecumenical Patriarch becomes Emperor after a successful overthrow revolt.
It is funny in the beginning but the purple blob becomes super stable and never returns to a normal feudal realm. I never had it in all my 2.5 games so far but a similar thread pops up every month.Bug or not I really like it. I hope they don't ever fix it. One day seeing that the whole Orthodox church took over the Byzantine Empire are those odd twists I play Crusader Kings II for!
Anyway thanks for the replies!