In its first part the article entitled „Historical Dimension of the Encyclical Ut Unum Sint of John Paul II in the Ecumenical Dialogue” presents the etymology of the appellation „ecumenism”, as well as its historical, geographic and theological significance. It also brings closer to the readers two different, but, in the same time, interrelated levels of perception of the term „ecumeny” in today’s Christianity. The second part of the work features specificity of the – over twenty six years long – pontificate of John Paul II, emphasizing his involvement in unifying and interreligious dialogue in all its dimensions (spiritual, doctrinal and practical). The third part of the paper consists of detailed analysis of content of all parts of the encyclical „Ut Unum Sint” (three chapters and the exhortation). In the final part of article readers can find examples of development of the present understanding of Roman Catholic definition of Christian unity.