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.

Although western medieval thinkers in general had a little interest in non-Christian religious traditions, one can find a sort of comparative study of religions already in the 13th century. As a rule, the purpose of such studies was to prove the superiority of Christianity over other religions, and the means used were often the instruments of ‘natural human reason’. Works of Roger Bacon (1214/22–ca. 1292) is the most striking example of this approach.