Judging by the historical records, by approximately 1000 AD the predominant ethnic group over much of modern European Russia, Ukraine and Belarus was the Eastern branch of the Slavs, speaking a closely related group of dialects. The political unification of this region into Kievan Rus' in about 880, from which modern Russia, Ukraine and Belarus trace their origins, established Old East Slavic as a literary and commercial language. It was soon followed by the adoption of Christianity in 988 and the introduction of the South Slavic Old Church Slavonic as the liturgical and official language. Borrowings and calques from Byzantine Greek began to enter the Old East Slavic and spoken dialects at this time, which in their turn modified the Old Church Slavonic as well.
What countries did Kievan Rus' become?
modern Russia, Ukraine and Belarus