How did the Byzantine emperor become the spiritual leader of most eastern Christians?

Explanation:

In 330 A.D., Roman Emperor Constantine I chose Byzantium as the site of a “New Rome” with an eponymous capital city, Constantinople. Five years earlier, at the Council of Nicaea, Constantine had established Christianity — once an obscure Jewish sect — as Rome's official religion.

the treaty of brest-litovsk was a peace treaty signed on 3 march 1918 between the new bolshevik government of russia and the central powers (german empire, austria-hungary, bulgaria, and the ottoman empire), that ended russia's participation in world war i.