When political relations between the Polish-Lithuanian state and the Duchy of Muscovy were in a diplomatic stalemate, the Cracow theologian Jan Sacranus of Oswiecim (1443-1527) created conceptual foundations for a model of the union between the Roman Catholic Church and the Orthodox Church. His treatise 'Elucidarius errorum ritus Ruthenici', written in 1500 and later published in print in Cracow and Cologne, is a unique document of religious polemic in the whole long-lasting controversy. He attempted to prove with historical and philosophical arguments that there is only one path for the Orthodox Church - that of uniting with the Roman Catholic Church. There was no place left for any unite form of church. Thus Sacranus paved the way for the Catholic side to communicate with the Orthodox side (and later the Unite one) which proved to be not only attractive in the intellectual sense, but also became a kind of leitmotif of the policy of both the Papacy and the Polish-Lithuanian state towards the Orthodox Church and the Florentine union. The main theses of Sacranus' treatise were later incorporated into both the Jesuit and the Calvinist strategies of attracting the Orthodox states of Europe and were applied in new political situations.