Slavic languages began differentiating from 6th century AD when Slav people migrated. By the 10th century AD, Western, Southern and Eastern Slavic languages had formed. Eastern Slavic was spoken in the territories of present-day Russia, Belarus and Ukraine. These were unified under Kievan Rus at the end of the 9th century. This established Old East Slavic as a commercial and literary language.
With Christianity, South Slavic Old Church Slavonic was introduced as the liturgical and official language at the end of the 10th century.
Differentiation of the Eastern Slavic language accelerated after the breakup of Kievan Rus in approximately 1100 AD. Russian, Belarusian and Ukrainian became more distinct languages around the 13th century.
The official language in Novgorod and Moscow until the 17th century was Church Slavonic, which developed from Old Church Slavonic. After the 17th century, the usage of Church Slavonic shrank drastically and was limited to only liturgical and biblical texts.
In the middle of the 18th century, in an attempt to standardise the written language, M.V.Lomonosov developed three distinct styles of Russian: High, Middle and Low. High style was used for poetry and religion and referred to as Old Church Slavonic. Middle was meant for science texts, prose, and Low style was used for personal correspondence.
Russian writer Alexander Pushkin contributed to efforts to move the Russian language away from Old Church Slavonic and develop a distinct Russian literary language in the late 18th and beginning of the 19th century. He rejected archaic grammar and vocabulary and used grammar and vocabulary of the spoken language.
More reforms to simplify the language were implemented after the revolution in 1917, when Russian assumed the modern form as we know it today.