The article focuses on the interpretation and use of the concepts “unia” and “schizma” by the Orthodox religious polemicists in the Polish-Lithuanian Commonwealth of the late 16th–17th century. These concepts became pivotal for the religious life of the Polish-Lithuanian Commonwealth after the Brest Church Union in 1595–1596, when the hierarchs of the Kyiv Metropolitanate recognized the authority of Rome. The concept “unia” for the Orthodox had purely negative connotations, denoting a violent and unnatural unification of churches not by the will of God, but by the will of infidels and willful people, which is why, playing on the similar sounding, the Orthodox spoke not about the “uniya” (Eng. “union”) of churches, but about the “mania” or “madness” of those who supported the “union” with Rome. It is natural that they were sure that it was the “unia” that caused all the religious conflicts in the Polish-Lithuanian Commonwealth at that time. The Orthodox authors also considered “unia” a betrayal of Orthodoxy, almost as a full synonym of “otstupnichestvo” (Eng. “apostasy”). Denying all accusations of “schizma” (Eng. “schism”) from Catholics and Uniates, the Orthodox noted that they could not be “schismatics” in any way, since they remained faithful to Orthodoxy. They considered the Uniates and Catholics to be the “true” “schismatics”, who, as the Orthodox believed, broke away from the one universal church and the Orthodox patriarchs, becoming proud and placing themselves above all other Christians. The author concludes that such interpretations and uses of the concepts “unia” and “schizma” clearly reflected the position of representatives of the Orthodox Church regarding “reunification” with Catholics. In fact, the Orthodox, unlike the Catholics and Uniates, insisted on the restoration of relations based on equality, mutual respect, and mutual love between the Eastern Orthodox Patriarchs and the Roman Pontiff.