Old Slavonic is not a religion, it’s the original language that Ukrainian Catholic and Ukrainian Orthodox Church’s used in liturgy.
Slavic is the region and people. Old Slavonic is the original language.
Hate to break it to you, Kieven-Rus is present day Ukraine.
It’s a rite, Old Slavonic. The language of Old Slavonic is Slavic. And yes, the fact that Kieven Rus is in present-day Ukraine should make my point that Ukrainians and Russians used to speak the same language (prior to the Polonization), no?