The subject of the monograph is a polemic-apologetical manuscript year-book, which is located in the National Library of Ukraine, Institute of Manuscripts, collection № 312, shelf mark Соф 104 (105). The manuscript is written in semi-uncial Cyrillic script in the Church Slavonic language. It is a late 16th century year-book against Protestantism concerning questions about the Eucharist, transubstantiation and liturgy. The preface of the manuscript is explaining the aim of compiling the year-book. It is interesting for its ecclesiology, especially because it illustrates the relation between Byzantine Christians and Latin Catholics in the Polish-Lithuanian commonwealth. The questions of Ecclesial affiliation of the author, his theological background as well as the date of composing the preface are also examined. The manuscript contains also a rare liturgy – the Liturgy of St. Peter. The structure of the liturgy is studied with the aim to state more precisely the process of its compilation. The analysis of the text of the liturgy, its structure, environment and manuscript tradition is also provided in order to determine whether it belongs to the Byzantine or the Latin tradition. It is one of the four preserved Slavonic manuscripts with this liturgy; the three other manuscripts are also briefly examined and compared in the monograph. The question about a possible relation of the Liturgy of St. Peter to the Cyrillo-Methodian mission is examined as well as the question of the date of its Slavonic translation.