^Sally Bruyneel; Alan G. Padgett. Introducing Christianity. Orbis Books. 2003: 7 [2 September 2013]. ISBN978-1-60833-134-5. "The Eastern Orthodox and thye Roman Catholic Churches are the oldest with roots going back to the earliest Christian groups."
^Robert L. Plummer. Journeys of Faith: Evangelicalism, Eastern Orthodoxy, Catholicism and Anglicanism. Zondervan. 6 March 2012: 128 [2 September 2013]. ISBN978-0-310-41671-5. "Catholicism holds that if a Church claims to be Christian, then it must be able to show that its leaders-its bishops and its presbyters (or priests)- are successors of the apostles. That is why the Catholic Church accepts Eastern Orthodox ordinations and sacraments as valid, even though Eastern Orthodoxy is not in full communion with Rome."
^William A. Dyrness; Veli-Matti Kärkkäinen. Global Dictionary of Theology: A Resource for the Worldwide Church. InterVarsity Press. 25 September 2009: 244 [2 September 2013]. ISBN978-0-8308-7811-6. "This connection is apparent through the historical succession of bishops of churches in a particular geographic locale and by fidelity to the teachings of the apostles (cf. Acts 2:42) and life as it developed in the patristic tradition and was articulated by the seven ecumenical councils."
^Heidi A Campbell. When Religion Meets New Media. Routledge. 22 March 2010: 13 [2 September 2013]. ISBN978-0-203-69537-1. "There are three branches within Christianity: Catholic, Eastern Orthodox, and Protestant. ... The Christian church draws its lineage and roots from the time of Jesus Christ and the apostles in CE 25–30 and the birth of the Church at Pentecost in ..."