The article investigates how, in the context of the controversy over the Brest Church Union of 1595–1596, the Orthodox, Roman Catholics and Uniates in the Polish-Lithuanian Commonwealth interpreted and used the concepts of “catholic” and “orthodox”. It is noted that supporters and opponents of the union with Rome considered their church to be both “catholic” and “orthodox”, fundamentally denying this to their counterparts. The author concludes that if the concept of “orthodox” was interpreted by Roman Catholics, Uniates and Orthodox alike, referring to the importance of preserving the purity of faith and following traditions of apostolic church, as representatives of each confession understood it, then the concept of “catholic” was interpreted by them differently. For Roman Catholics and Uniates, within the framework of this concept, the aspect associated with the spread of a particular church and religion throughout the world among all peoples was more important. For Orthodox, more important was preservation of the interconnectedness and complementarity of each constituent part of the church, as well as their equality in relation to each other and God. Thus, difference in interpretation of these most important ecclesiological concepts reflected not only the entire difference in views of Roman Catholics, Uniates and Orthodox on the structure of the Christian Church, but also position of these groups in the Polish-Lithuanian Commonwealth during the 17th century: from the “militant” impulse of Counter-Reformation Catholicism to “defensive” position taken by Orthodox.