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