Abstract: The Church Union of Brest (1595-1596) resulted from sincere aspiration of both, Catholic and Orthodox Churches, to overcome the schism. However, during the preliminary negotiations and in the very moment when the union was proclaimed, it became apparent that two counterparts had very divergent visions of ecclesiology, as we can see from documents and correspondence that have been analysed in this article. These sources revealed a rather acute conflict of two manners to understand ecclesiological aspects of the union to come. This situation might be regarded as a cultural misunderstanding between Catholic and Orthodox traditions too.