What is the Ruthenian language? A simple answer is that it was the precursor to modern Ukrainian and Belarusian. But the history and synchronic realization of this language are much more complex. During the sixteenth and seventeenth centuries Ruthenian was the language of the Orthodox and Uniate inhabitants of the Polish—Lithuanian Commonwealth. It enjoyed a culture and prestige greater than that of its Muscovite (Russian) neighbor to the east, but was often denigrated in comparison with the dominant language—Polish—of the Commonwealth.In Testament to Ruthenian, Stefan Pugh analyzes the Ruthenian language use of one of its most outstanding practitioners, Meletij Smotryc´kyj (ca. 1578–1633): polemicist, cleric, and scholar. This is the first study to survey comprehensively a Ruthenian speaker’s oeuvre, and it will provide the groundwork for the next generation of scholarship on the Ruthenian language.