The year 1643 saw the publication of Listy swięteo oyca Partheniusza do Piotra Mohily, which came out of the printing press of the Polish-language department of the Kyiv Lavra. In the Letters Parthenius – a relatively unknown patriarch of Constantinople – discusses Confessio Fidei – a succinct confession of faith published in Geneva in 1629 under the name of Cyril Lucaris – an Ecumenical Patriarch of Constantinople and opponent of the union. As Confessio is clearly Calvinist is spirit, Parthenius refutes Lucaris’ authorship and imposes an anathema on the genuine author of Confessio and its propagators. The recipient and a probable translator of Letters is Peter Mohyla – a distinguished Metropolitan of Kyiv, author of the first Orthodox catechism and founder of the Mohyla Collegium. The paper addresses the issue of the Letters, in particular their Polish translation and the identity of the translator but does so against the broader background of the circumstances occasioned by the emergence of the contentious Confessio and the Orthodox Church’s ultimate reaction to its emergence.