Background. In the article it is analyzed the dedication and preface to the work of the little-known in Ukraine polemicist-theologian of the 17-th century Theodore Skumynovych "Przyczyny porzucenia Disuniey przezacnemu narodowi ruskiemu podane" (1643), which is considered a bibliographic rarity. The relevance of the research of the creative work of Theodore Skumynovych is explained by the fact that it has not been studied in detail. Methods. The research uses cultural-historical, comparative-historical, descriptive and textological methods of analysis of monuments of literature. Results. The publication provides biographical information about Theodore Skumynovych, examines the objectivity of the accusations brought against him by former co-religionists, and outlines the main ideas of his polemical work. In the dedication of the work to the bishop of Vilnius Abraham Woyna and the foreword to the reader, Theodore Skumynovych briefly spoke about the reasons for his conversion to Catholicism, shared the details of his move from Volhynia to Vilnius and the difficulties he had to face. The image of Theodore Skumynovych, which appears in the introductory part of the work, contrasts with that how he is negatively presented in the complaints made by his opponents in the judicial books, which gives reason to question the credibility of the accusations made by them. The article clarifies the constants of theological and writing thinking of Theodore Skumynovych, reveals his point of view on the reasons of the decline of Byzantium. Conclusions. According to Kasiyan Sakovych, Theodore Skumynovych belonged to the most educated theologians of Ukraine in the 17-th century. Before converting to Catholicism, he was the Father Superior of the Orthodox St. Michael Monastery in Goshcha, the rector of the local school, and also a member of the metropolitan consistory. As an Orthodox clergyman, Theodore Skumynovych sharply criticized the dogmatics of the Roman Church. However, after reading the works of Patriarch Gennadius Scholarius, in which he explained the conquest of Constantinople by the Ottomans with the "Great Schism", reflecting on the Creed of Athanasius the Great, studying the works of the Greek fathers of the church and observing the crisis of monastic and church life in the Kyiv's Metropolitanate, he changed his opinion about the Catholics. The books written by Theodore Skumynovych were highly rated among famous scientists of Western Europe. In particular, his works were admired by Gottfried Leibniz, Jacob Masen and Plempius Vopisk Fortunatus.
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