According to this central mystery of most Christian faiths, there is only one God in three Persons: while distinct in their relations with each other ("it is the Father who generates, the Son who is begotten, and the Holy Spirit who proceeds"), they are stated to be one in all else, co-equal, co-eternal and consubstantial, and each is God, whole and entire. Accordingly, the whole work of creation and grace in Christianity is seen as a single operation common to all three divine persons, in which each shows forth what is proper to him in the Trinity, so that all things are "from the Father", "through the Son" and "in the Holy Spirit". C.S. Lewis makes the analogy to a cube and its six square faces: God is like the solid mass of the cube, invisible inside it, while the three Persons are like the squares, which are each equally its visible faces.
Trinitarian theologians believe that manifestations of the Trinity are made evident from the very beginning of the Bible. Genesis 1:1-3 posits God, His Spirit and the "creative word of God" together in the initial Genesis creation narrative account. While the Fathers of the Church saw Old Testament elements such as the appearance of three men to Abraham in Book of Genesis, chapter 18, as foreshadowings of the Trinity, it was the New Testament that they saw as a basis for developing the concept of the Trinity. One of the most influential of the New Testament texts seen as implying the teaching of the Trinity was 28:19, which mandated baptizing "in the name of the Father, and of the Son, and of the Holy Spirit". Another New Testament text pointing to the Trinity was (Revelation 1:8)
Reflection, proclamation, and dialogue led to the formulation of the doctrine that was felt to correspond to the data in the Bible.
While scripture does not contain the word Trinity, an indication of three distinct persons can be found in 1 John 5:7 for the validity of which exist a controversy known as Johannine Comma. Early Christian belief in the deity of Jesus Christ existed since the first century in the writings of 20:28), Hebrews 1:8-10), 2 Peter 1:1), as well as in the writings of Ignatius of Antioch, a disciple of John who was born about the beginning of the Apostolic age (c. 35). Jesus is also quoted as attesting to being one with and equal with the Father, sharing in the glory of the Father before the world began. (John 8:58, 10:30, 17:5).
Romans 8:9-11 implies the interdependency or interrelatedness of Father, Son, and Holy Spirit, while examining the redeemed individual, saved by grace, as evidenced by the indwelling Spirit of God.
Subsequently, in the understanding of Trinitarian theology, Scripture "bears witness to" the activity of a God who can only be understood in Trinitarian terms. The doctrine did not take its definitive shape until late in the fourth century. During the intervening period, various tentative solutions, some more and some less satisfactory, were proposed.
Trinitarianism contrasts with positions such as Binitarianism (one deity in two persons, or two deities) and Monarchianism (no pluratity of persons within God), of which Modalistic Monarchianism (one deity revealed in three modes) and Unitarianism (one deity in one person) are subsets. Additionally, the Church of Jesus Christ of Latter-day Saints believes the Father, the Son, and the Holy Ghost are three separate deities, two of which possess separate bodies of flesh and bones, while the Holy Ghost has only a body of spirit; and that their unity is not physical, but in purpose.
The word "trinity" is derived from Latintrinitas, meaning "the number three, a triad, tri". This abstract noun is formed from the adjective trinus (three each, threefold, triple), as the word unitas is the abstract noun formed from unus (one).
The corresponding word in Greek is tριάς, meaning "a set of three" or "the number three". The first recorded use of this Greek word in Christian theology was by Theophilus of Antioch in about the year of 170. He wrote:
In like manner also the three days which were before the luminaries, are types of the Trinity [Τριάδος], of God, and His Word, and His wisdom. And the fourth is the type of man, who needs light, that so there may be God, the Word, wisdom, man.
Tertullian, a Latin theologian who wrote in the early 3rd century, is credited as being the first to use the Latin words "Trinity", "person" and "substance" to explain that the Father, Son, and Holy Spirit are "tres personae, una substantia". While "personae" is often translated as "persons," the Latin word personae is better understood as referring to roles as opposed to individual centers of consciousness.