The European Union, in its constitutional project, had abandoned the idea of setting up Christianism as the foundation of a common identity. The reason of this choice was not only the distinction within the Christian religion between divergent forms, but also, during the long history of Europe, the presence of other religions, although in minority in this part of the world. Amongst them, Islam: «The tragical events in Bosnia showed the general public the existence, at the gates of western Europe, of a Slavonic-speaking Moslem population come from an ill-understood past and an almost unknown religious process: the past is that of Ottoman Rumelia; the process that of the conversion to Islam of some Balkanic peoples between the end of the Middle Ages and our time». Where does European Islam come from, be it Slavonic, Albanian or Türk, when it is older than the recent immigrations? The answer seems obvious: from the Ottoman Empire, when Istanbul replaced Constantinople. The Islamisation came either with the transportation of settlers from Anatolia or with the conversion of the natives to the new religion of the State. But how did this conversion come about? Its precedents are here observed, the existence in the Balkans since the Middle Ages of a faith which its believers told was Christian, even «true Christian», but which the contemporaneous clergymen branded as Manichean. The relation between this faith, in its various assemblies, and European Islam has been disputed. This study tries to present some elements for the debate. Beyond, will be left to ask what makes unity or division in human societies. For, notwithstanding the term, Balkanisation is not special to the Balkans.