In the histories of ecumenism, its initial formation is usually dated by the early 20^th century. The World Missionary Conference held in Edinburgh in 1910 is referred to as its “symbolic beginning”. A quest for the origins of the ecumenical thought led researchers to find some early voices in the previous centuries, even as early as in the 15^th–16^th c. However, there are Oriental sources which witness a much earlier formation of the ecumenical paradigm of the ecclesiological thought, typologically corresponding to the one developed in the 20^th c. In the Golden Age of Medieval Muslim culture under the ʿAbbasid caliphate, an ecumenical position is witnessed by some Middle Eastern Christian authors. In their works, the main Christian denominations are not polemically presented as opposed to each other, but on the contrary, the essential unity of various Christian beliefs is emphasized, and the ways the main Christian communities follow are claimed to be equal in value. The present article uses the Medieval Arabic sources to demonstrate that the history of the ecumenical thought should be corrected by supplying a chapter on the Medieval Eastern period of the history.