There is no "Old Russian" language. You are talking about Old Church Slavonic language which is created by Saints Cyril and Methodius to translate Bible.

https://en.wikipedia.org/wiki/Old_Church_Slavonic

PS.

Quote from (English) Wikipedia:

The official language in Moscow and Novgorod, and later, in the growing Muscovy, was Church Slavonic, which evolved from Old Church Slavonic and remained the literary language for centuries, until the Petrine age, when its usage became limited to biblical and liturgical texts. Russian developed under a strong influence of Church Slavonic until the close of the 17th century; afterward the influence reversed, leading to corruption of liturgical texts.

https://en.wikipedia.org/wiki/Russian_language

But Latin had a (or really many depending on era and style) written form and a spoken form (vulgate). My understanding is that the vulgate is quite similar to some Italian regional dialects, although not to standard Italian (itself a postwar construct)