Christian Church

===Christianity as Roman state religion===
{{Main|Christianity as the Roman state religion}}
[[File:Nicaea icon.jpg|thumb|An icon depicting [[Constantine I]], accompanied by the [[bishop]]s of the [[First Council of Nicaea]] (325), holding the [[Niceno-Constantinopolitan Creed|Niceno–Constantinopolitan Creed of 381]].]]
On February 27, 380, the Roman Empire officially adopted the Nicene version of Christianity as [[state church of the Roman Empire|its state religion]]. Prior to this date, [[Constantius II#Christianity under Constantius|Constantius II]] (337–361) and [[Valens#Struggles with the religious nature of the Empire|Valens]] (364–378) had personally favored Arian or [[Semi-Arianism|Semi-Arian]] forms of Christianity, but Valens' successor [[Theodosius I]] supported the more [[Athanasian]] or Trinitarian doctrine as expounded in the [[Nicene Creed]] from the 1st Council of Nicaea.

On this date, Theodosius I decreed that only the followers of Trinitarian Christianity were entitled to be referred to as [[Catholic (term)|Catholic Christians]], while all others were to be considered to be heretics, which was considered illegal.<ref name="TheodosiusDecree">{{cite web |title=Theodosian Code XVI.i.2 |url=http://www.fordham.edu/halsall/source/theodcodeXVI.html |work=Medieval Sourcebook: Banning of Other Religions |first=Paul |last=Halsall |date=June 1997 |publisher=Fordham University |access-date=2006-11-23 |archive-date=2007-02-27 |archive-url=https://web.archive.org/web/20070227120555/http://www.fordham.edu/halsall/source/theodcodeXVI.html |url-status=dead }}</ref> In 385, this new legal situation resulted, in the first case of many to come, in the capital punishment of a heretic, namely [[Priscillian]], condemned to death, with several of his followers, by a civil tribunal for the crime of magic.<ref>{{CathEncy|wstitle=Priscillianism|first=Patrick|last=Healy}}</ref> In the centuries of state-sponsored Christianity that followed, [[Paganism|pagans]] and heretical Christians were routinely persecuted by the Empire and the many kingdoms and countries that later occupied its place,<ref>[[Ramsay MacMullen]], ''Christianity and Paganism in the Fourth to Eighth Centuries'', Yale University Press, September 23, 1997</ref> but some [[Germanic peoples|Germanic tribes]] remained Arian well into the [[Middle Ages]]<ref>''Christianity Missions and monasticism'', Encyclopædia Britannica Online [https://www.britannica.com/eb/article-67441/Christianity]</ref> (see also [[Christendom]]).

The Church within the Roman Empire was organized under [[metropolitan sees]], with five rising to particular prominence and forming the basis for the [[Pentarchy]] proposed by [[Justinian I]]. Of these five, one was in the [[Western Roman Empire|West]] ([[Diocese of Rome|Rome]]) and the rest in the [[Eastern Roman Empire|East]] ([[Constantinople]], [[History of Jerusalem|Jerusalem]], [[Antioch]], and [[Alexandria]]).<ref>Deno Geanakoplos, ''A short history of the ecumenical patriarchate of Constantinople'', Archons of the Ecumenical Patriarch, retrieved May 20, 2007 [http://www.archons.org/patriarchate/history/pentarchy.asp]</ref>
[[File:Mor-mattai.png|thumb|230px|left|Founded in AD 363, [[Mar Mattai Monastery]], a [[Nestorian Church]], is recognized as one of the oldest Christian monasteries in existence.<ref>{{cite web|url=http://www.syriacstudies.com/AFSS/Syriac_Articles_in_English/Entries/2012/4/28_The_Christians_Under_Turkish_RuleCrusades_Conflict_Between_Christendom_And_Islam_Dr._Matti_Moosa.html|title=The Christians Under Turkish Rule|last=Moosa|first=Matti|date=28 April 2012}}</ref>]]
Even after the [[Roman Empire#Crisis of the Third Century and the later emperors (235–395)|split of the Roman Empire]] the Church remained a relatively united institution (apart from [[Oriental Orthodoxy]] and some other groups which separated from the rest of the state-sanctioned Church earlier). The Church came to be a central and defining institution of the Empire, especially in the East or [[Byzantine Empire]], where Constantinople came to be seen as the center of the Christian world, owing in great part to its economic and political power.<ref>{{cite encyclopedia |title=MSN Encarta: Orthodox Church, retrieved May 12, 2007 |url=http://encarta.msn.com/encyclopedia_761572657_6/Orthodox_Church.html |archive-url=https://web.archive.org/web/20091028211131/http://encarta.msn.com/encyclopedia_761572657_6/Orthodox_Church.html |archive-date=2009-10-28 |url-status=dead }}</ref><ref>Arias of Study: Western Art, Department of Art History, University of Wisconsin, retrieved May 17, 2007 [http://www.uwm.edu/Dept/ArtHistory/areas_study.html]</ref>

Once the [[Fall of Rome|Western Empire fell]] to Germanic incursions in the [[Christianity in the 5th century|5th century]], the (Roman) Church became for centuries the primary link to [[Culture of ancient Rome|Roman civilization]] for medieval [[Western Europe]] and an important channel of influence in the West for the Eastern Roman, or ''Byzantine'', emperors. While, in the West, the so-called [[orthodoxy|orthodox]] Church competed against the Arian Christian and pagan faiths of the Germanic rulers and spread outside what had been the Empire to Ireland, Germany, Scandinavia, and the western [[Slavs]], in the East Christianity spread to the Slavs in what is now [[Russia]], south-central and eastern Europe.<ref name="CHRISTIANITY IN HISTORY">''CHRISTIANITY IN HISTORY'', Dictionary of the History of Ideas, University of Virginia Library [http://etext.lib.virginia.edu/cgi-local/DHI/dhi.cgi?id=dv1-49] {{Webarchive|url=https://web.archive.org/web/20060909175126/http://etext.lib.virginia.edu/cgi-local/DHI/dhi.cgi?id=dv1-49|date=2006-09-09}}</ref>

Starting in the [[Christianity in the 7th century|7th century]], the [[Islamic Caliphate]]s rose and gradually began to conquer larger and larger areas of the [[Christendom|Christian world]].<ref name="CHRISTIANITY IN HISTORY" /> Excepting [[North Africa]] and [[Al-Andalus|most of Spain]], northern and western Europe escaped largely unscathed by Islamic expansion, in great part because richer Constantinople and its empire acted as a magnet for the onslaught.<ref>{{Cite web |url=http://www.byzantinos.com/Byzantium/Legacy.html |title=''The Byzantine Empire'', byzantinos.com |access-date=2007-05-24 |archive-url=https://web.archive.org/web/20070928042403/http://www.byzantinos.com/Byzantium/Legacy.html |archive-date=2007-09-28 |url-status=dead }}</ref> The challenge presented by the Muslims would help to solidify the religious identity of eastern Christians even as it gradually weakened the Eastern Empire.<ref>''BYZANTINE ICONOCLASM AND POLITICAL EARTHQUAKE OF ARAB CONQUESTS – AN EMOTIONAL 'GUST''', This Century's Review, retrieved May 24, 2007 [http://www.thiscenturyreview.com/BYZANTINE_ICONOCLASM_AND_POLITICAL.byzantineiconoclasm.0.html]</ref> Even in the [[Muslim World]], the Church survived (e.g., the modern [[Copt]]s, [[Maronite]]s, and others) albeit at times with great difficulty.<ref>''The History of the Copts'', California Academy of Sciences {{cite web |url=http://www.calacademy.org/research/anthropology/coptic/Copthist.htm |title=Coptic History |access-date=2007-10-28 |url-status=dead |archive-url=https://web.archive.org/web/20071013191715/http://calacademy.org/research/anthropology/coptic/Copthist.htm |archive-date=2007-10-13 }}, retrieved May 24, 2007</ref><ref>''History of the Maronite Patriarchate'', Opus Libani, retrieved May 24, 2007 {{cite web |url=http://www.opuslibani.org.lb/egliseeng/002/antioch1.html |title=History of the Maronite Patriarchate |access-date=2007-10-28 |url-status=dead |archive-url=https://web.archive.org/web/20071013194405/http://opuslibani.org.lb/egliseeng/002/antioch1.html |archive-date=2007-10-13 }}</ref>