All these languages are nowadays separate in their own right. Until the 17th century it was usual to call Belarussian ("White Russian"), Ukrainian ("Little Russian"), Russian ("Great Russian") dialects of one common "Russian" language (the common languages of Eastern Slavic countries). Despite the vast territory occupied by the East Slavs, their languages are astonishingly similar to one another, with transitional dialects in border regions. All these languages use the Cyrillic alphabet, but with particular modifications.
The history of the East Slavic languages is a very 'hot' subject, because it is interpreted from various political perspectives by the East Slavs "like all mortals, wishing to have an origin as ancient as possible" ("sicut ceteri mortalium, originem suam quam vetustissimam ostendere cupientes"), as Aeneas Sylvius observed in his Historia Bohemica in 1458.
Therefore, a crucial differentiation has to be made between the history of the East Slavic dialects and that of the literary languages employed by the Eastern Slavs. Although most ancient texts betray the dialect their author(s) and/or scribe(s) spoke, it is also clearly visible that they tried to write in a language different from their dialects and to avoid those mistakes that enable us nowadays to locate them.
In both cases one has to keep in mind that the history of the East Slavic languages is of course a history of written texts. We do not know how the writers of the preserved texts would have spoken in every-day life, let alone how an illiterate East Slavic peasant spoke to his family.
After the conversion of the East Slavic region to Christianity the people used service books borrowed from Bulgaria, which were written in "Old Bulgarian" or Old Church Slavonic. They continued to use this language, or rather a variant thereof, usually called (Middle) Church Slavonic, not only in liturgy, but also generally as the language of learning and written communication. This left a large imprint even on the rare secular texts.
Throughout the Middle Ages (and in some way up to the present day) there existed a duality between the Church Slavonic language used as some kind of 'higher' register (not only) in religious texts and the popular tongue used as a 'lower' register for secular texts. It has been suggested to describe this situation as diglossia, although there do exist mixed texts where it is sometimes very hard to determine why a given author used a popular or a Church Slavonic form in a given context. Church Slavonic was a major factor in the evolution of modern Russian, where there still exists a "high stratum" of words that were imported from this language.
Rozmova. Beseda. das Ruthenische Und Kirchenslavische Berlaimont-Gesprächsbuch Des Ivan Uzevyc. Mit Lateinischem Und Polnischem Paralleltext/ Die Ruthenische Schriftsprache Bei Ivan Uzevyc Unter Besonderer Berücksichtigung der Lexik Seines Gesprächsbuchs «Rozmova/ Beseda». Mit Wörterverzeichnis Und Indizes Zu Seinem Ruthenischen Und Kirchenslavischen Gesamtwerk
Mar 01, 2007; Daniel Buncic and Helmut Keipert, eds. Rozmova. Beseda. Das ruthenische und kirchenslavische Berlaimont-GesprÃ¤chsbuch des Ivan...