SUMMARY: Peter Mohyla (c. 1596–1647) was the first legally appointed Metropolitan of Kiev, head of the Russian Orthodox Church in the Polish-Lithuanian State, restored in its rights in 1633, after a long period of pseudo-clandestine existence since the 1596 Union of Brest. In the 1630s and 1640s Peter Mohyla embarked on a series of ecclesiastical and educational reforms aimed at the revival of Russian Church, then in a state of crisis: its hierarchy corrupt, its priests ill-educated, its flock frustrated. His system of reforms was all-embracing and, in the long run, intended to change both the outlook and the image of the Russian Orthodox Church stagnating in its self-imposed isolation from European Christian culture, by cross-fertilization of Orthodox heritage with the achievements of Western civilization. In response to that urgent need, Mohyla created a system of education along Western lines, aimed at putting the Orthodox on a par with the post-Reformation Catholicism; he established printing presses and libraries of printed books, revised Orthodox liturgy and ritual, laid foundations of the system of Russian Orthodox theology, and made decisive steps towards the creation of Orthodox intellectual clerical elite. Mohyla’s westernizing influence was manifest in Russian culture until the latter part of the nineteenth century. Much has been written about Peter Mohyla. However, the first half of his life, the most important period when his character had been formed under the impact of the family traditions, education and possibly – travel, still largely remains in the dark. The article gives an outline of Peter Mohyla’s life, paying special attention to the previously unknown, overlooked or misinterpreted details related to his early life. Some of these details are derived from such somewhat untraditional historical sources as ownership or presentation inscriptions and other provenance evidence in books from Mohyla’s private library or those he donated to monasteries and churches in Galicia and the Kievan region. In addition, attempt is made to ‘revisit’ the question of Mohyla’s intellectual sources through the analysis of the surviving volumes from his library and two lists of books he purchased in 1632 and 1633 in Poland.