I was skeptical that Russia was even influenced much by the Eastern Orthodox Church before the Great Schism, but I looked it up and found that it was: before the Great Schism, the Rus tried to invade Byzantium, which increased trade and cultural exchanges, and an important Rus ruler, Vladimir, converted to Orthodox Christianity.
[1] https://en.wikipedia.org/wiki/East%E2%80%93West_Schism