God in Christianity

=== History and foundation ===
{{Further|Trinitarianism in the Church Fathers}}
In early Christianity, the concept of salvation was closely related to the invocation of the "Father, Son and Holy Spirit".<ref name="Vickers2"/><ref name=PCPhan3>''The Cambridge Companion to the Trinity'' by Peter C. Phan 2011 {{ISBN|0521701139}} pages 3–4</ref> Since the 1st century, Christians have called upon God with the name "Father, Son and Holy Spirit" in prayer, baptism, communion, exorcism, hymn-singing, preaching, confession, absolution and benediction.<ref name="Vickers2">Vickers, Jason E. ''Invocation and Assent: The Making and the Remaking of Trinitarian Theology.'' Wm. B. Eerdmans Publishing, 2008. {{ISBN|0-8028-6269-1}} pages 2–5</ref><ref name=PCPhan3/> This is reflected in the saying: "Before there was a 'doctrine' of the Trinity, Christian prayer invoked the Holy Trinity".<ref name="Vickers2"/>

[[File:Dogmatic sarcophagus.JPG|thumb|upright|left|The earliest known depiction of the Trinity, [[Dogmatic Sarcophagus]], 350 AD<ref>Elizabeth Lev, "Dimming the Pauline Spotlight; Jubilee Fruits" [http://www.zenit.org/article-26288?l=english Zenit 2009-06-25]</ref> [[Vatican Museums]].]]
The term "Trinity" does not explicitly appear in the Bible, but Trinitarians believe the concept as later developed is consistent with biblical teachings.<ref name=mercer935/><ref name=Kelly115>Kelly, J.N.D. ''Early Christian Doctrines'' A & C Black: 1965, p 115</ref> The [[New Testament]] includes a number of the usages of the three-fold liturgical and [[doxology|doxological]] formula, e.g., [[wikisource:Bible (American Standard)/2 Corinthians#1:21|2 Corinthians 1:21–22]] stating: "he that establisheth us with you in Christ, and anointed us, is God; who also sealed us, and gave [us] the earnest of the Spirit in our hearts".<ref name=mercer935/><ref name=RA122/> Christ receiving "authority and co-equal divinity" is mentioned in [[wikisource:Bible (American Standard)/Matthew#28:18|Matthew 28:18]]: "All authority hath been given unto me in heaven and on earth" as well as [[wikisource:Bible (American Standard)/John#3:35|John 3:35]], [[wikisource:Bible (American Standard)/John#13:3|John 13:3]], [[wikisource:Bible (American Standard)/John#17:1|John 17:1]].<ref name=RA122/> And the Spirit being both "of God" and "of Christ" appears in [[wikisource:Bible (American Standard)/Galatians#4:6|Galatians 4:6]], the [[Book of Acts]] ([[wikisource:Bible (American Standard)/Acts#16:7|16:7]]), [[wikisource:Bible (American Standard)/John#15:26|John 15:26]] and [[wikisource:Bible (American Standard)/Romans#8:14|Romans 8:14–17]].<ref name=RA122>[[Alan Richardson (priest)|Richardson, Alan]]. ''An Introduction to the Theology of the New Testament'' SCM: 1961, p122f,158</ref>

The general concept was expressed in early writings from the beginning of the 2nd century forward, with [[Irenaeus]] writing in his ''Against Heresies'' ([[Wikisource:Ante-Nicene Fathers/Volume I/IRENAEUS/Against Heresies: Book I/Chapter X.|Book I Chapter X]]):<ref name="Vickers2"/>

: "The Church ... believes in one God, the Father Almighty, Maker of heaven, and earth, and the sea, and all things that are in them; and in one Christ Jesus, the Son of God, who became incarnate for our salvation; and in the Holy Spirit".

Around AD 213 in ''Adversus Praxeas'' ([[Wikisource:Ante-Nicene Fathers/Volume III/Anti-Marcion/Against Praxeas/III|chapter 3]]) [[Tertullian]] provided a formal representation of the concept of the [[Trinity]], i.e., that God exists as one "substance" but three "Persons": The Father, the Son and the Holy Spirit.<ref name=Olson29>''The Trinity'' by Roger E. Olson, Christopher Alan Hall 2002 {{ISBN|0802848273}} pages 29–31</ref><ref>''Tertullian, First Theologian of the West'' by Eric Osborn (4 Dec 2003) {{ISBN|0521524954}} pages 116–117</ref> In defense of the coherence of the Trinity Tertullian wrote ([[Wikisource:Ante-Nicene Fathers/Volume III/Anti-Marcion/Against Praxeas/III|Adversus Praxeas 3]]): "The Unity which derives the Trinity out of its own self is so far from being destroyed, that it is actually supported by it." Tertullian also discussed how the Holy Spirit proceeds from the Father and the Son.<ref name=Olson29/>

The [[First Council of Nicaea]] in AD 325 and later the [[First Council of Constantinople]] in AD 381 defined the dogma "in its simplest outlines in the face of pressing [[heresy|heresies]]" and the version used thereafter dates to 381.<ref name=Fair48 >''Life in the Trinity: An Introduction to Theology with the Help of the Church Fathers'' by Donald Fairbairn (28 September 2009) {{ISBN|0830838732}} pages 48–50</ref> In the 5th century, in the [[Western Church|west]], [[Augustine of Hippo]] expanded on the theological development in his ''[[On the Trinity]]'', while the major development in the [[Eastern Church|east]] was due to [[John of Damascus]] in the 8th century.<ref name=berk83>''Systematic Theology'' by Louis Berkhof (24 September 1996) {{ISBN|0802838200}} page 83</ref> The theology eventually reached its classical form in the writings of [[Thomas Aquinas]] in the 13th century.<ref name=berk83/><ref name="ODCC-Trin" />

Bernhard Lohse (1928–1997) states that the doctrine of the Trinity does not go back to non-Christian sources such as [[Plato]] or [[Hinduism]] and that all attempts at suggesting such connections have floundered.<ref>''A Short History of Christian Doctrine'' by Bernhard Lohse 1978 {{ISBN|0800613414}} page 37</ref> The majority of Christians are now Trinitarian and regard belief in the Trinity as a test of true [[orthodoxy]] of belief.<ref name="Vickers2" />