Trinity

===Middle Ages===
Gregory of Nazianzus, Gregory of Nyssa, and Basil the Great account for the Trinity saw that the distinctions between the three divine persons were solely in their inner divine relations. There are not three gods, God is one divine Being in three persons.<ref>{{ cite book |last1=Shelley |first1=Bruce L. |title=Church History in Plain Language|pages= 113 |year=2013}}</ref> Where the Cappadocian Fathers used social analogies to describe the triune nature of God, Augustine of Hippo used psychological analogy. He believed that if man is created in the image of
God, he is created in the image of the Trinity. Augustine's analogy for the Trinity is the memory, intelligence, and will in the mind of a man. In short, Christians do not have to think of three persons when they think of God; they may think of one person.<ref>{{ cite book |last1=Shelley |first1=Bruce L. |title=Church History in Plain Language|pages= 113 |year=2013}}</ref>

In the late 6th century, some Latin-speaking churches added the words "and from the Son" ({{lang|la|[[Filioque]]}}) to the description of the procession of the Holy Spirit, words that were not included in the text by either the Council of Nicaea or that of Constantinople.<ref>For a different view, see e.g. [http://www.ccel.org/ccel/schaff/npnf214.x.xvi.xi.html Excursus on the Words πίστιν ἑτέραν]</ref> This was incorporated into the liturgical practice of Rome in 1014.<ref>{{cite book |title=Greek and Latin Traditions on Holy Spirit |url=http://www.ewtn.com/library/CURIA/PCCUFILQ.HTM |access-date=18 January 2019}}</ref> {{lang|la|Filioque}} eventually became one of the main causes for the [[East–West Schism]] in 1054, and the failures of the repeated union attempts.

Gregory of Nazianzus would say of the Trinity, "No sooner do I conceive of the One than I am illumined by the splendour of the Three; no sooner do I distinguish Three than I am carried back into the One. When I think of any of the Three, I think of Him as the Whole, and my eyes are filled, and the greater part of what I am thinking escapes me. I cannot grasp the greatness of that One so as to attribute a greater greatness to the rest. When I contemplate the Three together, I see but one torch, and cannot divide or measure out the undivided light."<ref>Gregory of Nazianzus, ''Orations'' 40.41</ref>

Devotion to the Trinity centered in the French monasteries at Tours and Aniane where [[Benedict of Aniane]] dedicated the abbey church to the Trinity in 872. Feast days were not instituted until 1091 at [[Cluny Abbey|Cluny]] and 1162 at Canterbury and papal resistance continued until 1331.{{sfn|Mulhern|1967|p=205}}