With the split of the Roman Empire in two split the church. Although not too much in confrontation which one was the right one. The antagonism developed latter. 

Example: The Byzantine church refused to crown Samuil as tzar because  Byzantine  was at war with the Slavs so it was done by the Roman Pope.  Since the Bulgarian crown was available he was crowned as Bulgarian tzar. By the title it was Bulgarian Empire but by the content it was Slavic Empire rooted in the land of my predecessors the Brzits an old Slavic tribe still existing which basically formed the Macedonian new nation.

btw the Berzits were core of the Dushan medieval Serbia but that did not prevent them to maintain their tribe independence and resist to the Bulgarian and Serbian attempts to be assimilated which came during the times of forming national states. On the contrary they stick with the few other Macedonian Slavic tribes and formed the Macedonian Slavic Nation.