Quote Originally Posted by Karavanshchik View Post
I'm always curious about this. Church Slavonic is the usual church language in Orthodox Churches in Eastern Europe and the Balkans. I know this is not a common question. How did Church Slavonic contributed to Russian idiomatic expressions, words and grammar?
Sometimes informally they call Old Church Slavonic "Old Bulgarian". It came from Slavic territories of former Byzantium with religion and missionaries (and with alphabet BTW). You need a specialist to have a responsible answer to your question though in my feeling situation is the following:

OCS gave a set of Russian idioms, lightly influenced literary Russian vocabulary and grammar and almost did not influence spoken Russian (outside of those idioms).