Here is the Statute of the Grand Duchy of Lithuania, written in its official language, Ruthenian in Cyrillic script
Old Belarusian was the official language of the Grand Duchy until the XVII century when it was replaced with Polish.
The language was called, as far as I have seen руски езыкъ or простый руский языкъ (not sure about spellings) "Rus' language" or "simple Rus' language," but in English it is usually called Chancery Slavonic
The reconstruction of nations: Poland, Ukraine, Lithuania, Belarus, 1569-1999 By Timothy Snyderhttp://books.google.com/books?id=QJhMhTKw-vgC&pg=PA19&lpg=PA19&dq=Chancery+Slavonic&source=bl&ots=lvWBHGgvF-&sig=anztiz1aOKGgrEm25yKOiC7zumU&hl=en&ei=LucMTuYHz4uwAtDWhb4K&sa=X&oi=book_result&ct=result&resnum=3&ved=0CCUQ6AEwAg#v=onepage&q=Chancery%20Slavonic&f=false
(btw, on p. 23 it states that the Statute of the Grand Duchy was written by a committee of 5 Orthodox and 5 communicants with the Vatican.)
This is the only visual I've gotten on the underlying languages of the time (most just show the states):
The Yellow is basically what went on to become Rusyn/Carpatho-Russian/Ruthenian; the Pink Urkainian, the light green Belarussian, the dark Green Russian. That only goes for the standard languages.
I say underlying because there was a diglossia between Old Church Slavonic, now in recensions of Church Slavonic which reflected the underlying vernacular. It was the vernacular in the areas which became the Belarussian dialects, upon which the Belarussian standard was based in the 19th into the 20th century, which basically developed from a recension of словѣ́ньскъ ѩзꙑ́къ into a руски езыкъ, i.e. an independent language. Ruthenian is the term borrowed into English via Latin, but yes, it would be Old Belarusian.