The main purpose of the study was the publication of a letter by Meletius Smotrytsky, the most prominent polemical writer of Ruthenian origin in the Polish-Lithuanian Commonwealth in the first half of the 17th century. This letter was previously unknown to the wider scholarly community. It is a beautiful testimony to the striving for unity among Eastern Rite Christians in the Polish-Lithuanian Commonwealth after the division that occurred between them as a result of the Union of Brest (1595/96). In addition, three letters have been published which deal with an important issue mentioned by Smotrycki in his letter to Chreptowicz, namely the discussions on the unification of the Uniates with the Orthodox Christians held at the end of 1623.