The Trinity is the term employed to signify the central doctrine of the Christian
                     religion -- the truth that in the unity of the Godhead there are Three Persons, the
                     Father, the Son, and the Holy Spirit, these Three Persons being truly distinct one
                     from another. Thus, in the words of the Athanasian Creed: "the Father is God,
                     the Son is God, and the Holy Spirit is God, and yet there are not three Gods but
                     one God." In this Trinity of Persons the Son is begotten of the Father by an
                     eternal generation, and the Holy Spirit proceeds by an eternal procession from
                     the Father and the Son. Yet, notwithstanding this difference as to origin, the
                     Persons are co-eternal and co-equal: all alike are uncreated and omnipotent.
                     This, the Church teaches, is the revelation regarding God's nature which Jesus
                     Christ, the Son of God, came upon earth to deliver to the world: and which she
                     proposes to man as the foundation of her whole dogmatic system.

                     In Scripture there is as yet no single term by which the Three Divine Persons are
                     denoted together. The word trias (of which the Latin trinitas is a translation) is
                     first found in Theophilus of Antioch about A.D. 180. He speaks of "the Trinity of
                     God [the Father], His Word and His Wisdom ("Ad. Autol.", II, 15). The term may,
                     of course, have been in use before his time. Afterwards it appears in its Latin
                     form of trinitas in Tertullian ("De pud." c. xxi). In the next century the word is in
                     general use. It is found in many passages of Origen ("In Ps. xvii", 15). The first
                     creed in which it appears is that of Origen's pupil, Gregory Thaumaturgus. In his
                     Ekthesis tes pisteos composed between 260 and 270, he writes:

                          There is therefore nothing created, nothing subject to another in the
                          Trinity: nor is there anything that has been added as though it once
                          had not existed, but had entered afterwards: therefore the Father
                          has never been without the Son, nor the Son without the Spirit: and
                          this same Trinity is immutable and unalterable forever (P. G., X,

                     It is manifest that a dogma so mysterious presupposes a Divine revelation. When
                     the fact of revelation, understood in its full sense as the speech of God to man, is
                     no longer admitted, the rejection of the doctrine follows as a necessary
                     consequence. For this reason it has no place in the Liberal Protestantism of
                     today. The writers of this school contend that the doctrine of the Trinity, as
                     professed by the Church, is not contained in the New Testament, but that it was
                     first formulated in the second century and received final approbation in the fourth,
                     as the result of the Arian and Macedonian controversies. In view of this assertion
                     it is necessary to consider in some detail the evidence afforded by Holy
                     Scripture. Attempts have been made recently to apply the more extreme theories
                     of comparative religion to the doctrine ot the Trinity, and to account for it by an
                     imaginary law of nature compelling men to group the objects of their worship in
                     threes. It seems needless to give more than a reference to these extravagant
                     views, which serious thinkers of every school reject as destitute of foundation.

The Catholic Encyclopedia:  NewAdvent.org