The language of 19th century (even its beginning), judging from the poetry and prose of Pushkin and Lermontov was quite the same language we speak today. Even if somebody decided to compose similar things today he would arrive to quite the same result.
The language of 18th century, that of Peter the Great and Lomonosov was quite different. It used many service words considered today archaic and most importantly, used a very different word order. While Russian language has a free word order, certain word order still considered the default, and this is the most prominent feature that distinguishes the language of 18th century. It is still well comprehensible and mostly remains grammatically valid.
But if to consider the language of 17th century, it is unlikely that a modern speaker would understand it without prior training. Actually it is a different language, comprehensible no more than modern Ukrainian.
I wonder what led to this dramatic change in Russian language in 17th century and also what caused the change in word order in the 18th century.