Historically, most Christian churches have taught that the nature of God is a mystery, in the original, technical meaning; something that must be revealed by special revelation rather than deduced through general revelation. Among early Christians there was considerable debate over the nature of Godhead, with some factions arguing for the deity of Jesus and others calling for an Arian conception of God. These issues of Christology were to form one of the main subjects of contention at the First Council of Nicaea.
The First Council of Nicaea, held in Nicaea in Bithynia (in present-day Turkey), convoked by the Roman Emperor Constantine I in 325, was the first (or second, if one counts the apostolic Council of Jerusalem) ecumenical council of bishops of the Catholic Church, and most significantly resulted in the first uniform Christian doctrine, called the Nicene Creed. With the creation of the creed, a precedent was established for subsequent 'general (ecumenical) councils of bishops' (synods) to create statements of belief and canons of doctrinal orthodoxy— the intent being to define unity of beliefs for the whole of Christendom and eradicate heretical ideas.
The purpose of the council was to resolve disagreements in the Church of Alexandria over the nature of Jesus in relationship to the Father; in particular, whether Jesus was of the same substance as God the Father or merely of similar substance. St. Alexander of Alexandria and Athanasius took the first position; the popular presbyter Arius, from whom the term Arian controversy comes, took the second. The council decided against the Arians overwhelmingly. (Of the estimated 250-318 attendees, all but 2 voted against Arius).
Christian orthodox traditions (Eastern Orthodox, Roman Catholic, Protestant, and Evangelical) follow this decision, which was codified in 381 and reached its full development through the work of the Cappadocian Fathers. They consider God to be a triune entity, called the Trinity, comprising the three "Persons" God the Father, God the Son, and God the Holy Spirit, the three of this unity are described as being "of the same substance" (ὁμοούσιος). The true nature of an infinite God, however, is asserted to be beyond definition, and "the word 'person' is but an imperfect expression of the idea, and is not biblical. In common parlance it denotes a separate rational and moral individual, possessed of self-consciousness, and conscious of his identity amid all changes. Experience teaches that where you have a person, you also have a distinct individual essence. Every person is a distinct and separate individual, in whom human nature is individualized. But in God there are no three individuals alongside of, and separate from, one another, but only personal self distinctions within the divine essence, which is not only generically, but also numerically, one."
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