Why did the two major regional versions of christianity part? how significant was the split?

Doctrinally, the split between the division of Christianity had to do with the nature of the deity. As a matter of historical causation, however, it is difficult to overlook the fact that the split in Christianity mirrored the concomitant division of the Roman Empire into Eastern and Western halves. As the halves of the empire grew further apart militarily and economically, so too did they grow further apart culturally and religiously.

It taught people how to live a good life, but not what to believe.