LOL! Old Slavonic is a religion. You mean Old Slavic. And since Russian comes from Kievan Rus, what you have said is otherwise true, but comically meaningless.
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.