Trinitarian

The Christian doctrine of the Trinity (Latin: Trinitas, lit. 'triad', from trinus, "threefold") holds that God is three consubstantial persons or hypostases—the Father, the Son (Jesus Christ), and the Holy Spirit—as "one God in three Divine Persons". The three persons are distinct, yet are one "substance, essence or nature" (homoousios). In this context, a "nature" is what one is, whereas a "person" is who one is.

According to this central mystery of most Christian faiths, there is only one God in three persons: while distinct from one another in their relations of origin (as the Fourth Lateran Council declared, "it is the Father who generates, the Son who is begotten, and the Holy Spirit who proceeds") and in their relations with one another, 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 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".

While the Fathers of the Church saw even 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 Matthew 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 John 1:1-14, in which the inter-relationships of the Triune God are reflected in the gospel author's description of "the Word", again showing the elements of the Triune God and their eternal (always was, always is, and always shall be) existence. Reflection, proclamation, and dialogue led to the formulation of the doctrine that was felt to correspond to the data in the Bible. The simplest outline of the doctrine was formulated in the 4th century, largely in terms of rejection of what was considered not to be consonant with general Christian belief. Further elaboration continued in the succeeding centuries.

...
Wikipedia