Orthodox Christianity is the second largest Christian communion in the world (250–300 million people), encompassing various national and regional churches that are united by their shared theological vision and sacramental practices while retaining their organizational independence.

Most of the original Orthodox Churches that trace their history back to the first centuries of the Christian era (e.g., the Patriarchates of Jerusalem and Constantinople; the Armenian Apostolic Church; and the national Church of Greece) are associated with the ethnic groups in the Middle East and Eastern Europe. However, the last three decades has witnessed a considerable increase in the Orthodox presence in North America, Western Europe, and Australia—often referred to as the “Orthodox diaspora”—through immigration and, increasingly, through conversion. These changes in the ethnic-cultural situation of the church, ...