This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book illustrates the challenges for Orthodox Christians to articulate theologically correct approaches to Islam and vice versa. It gives an account of the obstacles that the Patriarchate of Constantinople and the Greek Orthodox minority of Istanbul still face in Turkey, highlighting some recent positive developments of the Turkish authorities towards religious minorities in the light of the accession process of full EU membership. The book provides an account of nationalism and the establishment of nation-states in the Balkan Peninsula, which accordingly resulted in the establishment of national Churches. It also examines the foundation of the Church of Greece and its inextricable relations with the Ecumenical Patriarchate of Constantinople, along with the development of the Greek Church after the unilateral and un-canonical declaration of its autocephaly.