Yes the byzantines were the center of Orthodoxy. It was mostly due to them and the First Bulgarian Empire that Orthodoxy along with the cyrillic alphabet spread in Eastern, Northeastern Europe and Russia. I do not know much about the byzantines, but the bulgarians at that time invested a lot of money and resources in translating and spreading Christianity north and northeast. The tight trade connections and interactions between byzantines, bulgars and the various Rus and slavic states and the common slavic language also helped.

Source: mostly "History of Bulgaria", Bulgarian Academy of sciences,

Which countries are you thinking of, more specifically?