Trinity, THE DOCTRINE OF THE, is the highest and most mysterious doctrine of the Christian religion. It declares that there are three Persons in the Godhead, or divine nature—the Father, the Son, and the Holy Ghost, and that 'these three are one true, eternal God, the same in substance, equal in power and glory—although distinguished by their personal properties.' The most elaborate statement of the doctrine is to be found in the Athanasian Creed, which asserts that 'the Catholic faith is this: That we worship one God as Trinity, and Trinity in Unity—neither confounding the persons nor dividing the substance—for there is one person of the Father, another of the Son, and another of the Holy Ghost. But the Godhead of the Father, and of the Son, and of the Holy Ghost is all one; the glory equal; the majesty co-eternal.'
It is admitted that the doctrine is not found in its fully-developed form in the Scriptures; but it is believed to be clearly revealed in its elements in the New Testament, and also to be indicated in many of the statements and revelations of the Old Testament. The form of expression in speaking of God in the Old Testament Scriptures—the plural Elohim, coupled with a singular verb; the apparent distinction recognised in the revelations to the Patriarchs and Moses between Jehovah and 'the Angel of Jehovah,' the mode in which 'the Spirit' and 'Word' of God, and 'Wisdom' (Proverbs viii.) are spoken of; and the gradual unfolding of the doctrine of a 'Messiah,' have all been regarded as indications from the earliest times of the truth of a plurality of persons in the Godhead. In the New Testament Scriptures the doctrine is represented as clearly taught in the Trinitarian formula of baptism, the general character of the claims and prerogatives of Jesus Christ, especially the ascription to Him of the designation 'the Son of God,' and in the functions attributed to the Holy Spirit. The evidence is held conclusive of the equal divine nature and yet distinct personality of the Son and the Spirit along with God the Father. It is generally conceded, however, that the Christians of the 2d and even of the 3d century were far from having a clearly understood and recognised doctrine on this high subject. They were content for the most part to use scriptural expressions in speaking of the Father, and the Son, and the Spirit, without defining articulately their relation to one another. It was not till the progress of opposing heresies sought, on the one hand, to degrade the divine dignity of Christ, or, on the other, to confound the personality of Christ with God the Father, that the church was led to define in its Creeds, Nicene (325 A.D.) and others, the relation of the Son to the Father; the Christological controversies which led to the ultimate definition are discussed at CHRIST. The Constantinopolitan Creed (381) affirmed also the deity of the Holy Spirit; but this church doctrine of a Trinity of Persons was not fully completed till the addition of the Filioque clause in the Western Church of the 6th century, which led to the separation of the Greek Church from the Latin (see GREEK CHURCH, CHURCH HISTORY, CREEDS). The Western or Latin Church had less genius for such speculations, and, in so far as it meddled with them, imparted to them a coarser and more contradictory aspect—witness the so-called Athanasian Creed (q.v.). Strenuous assertion of the doctrine of the Trinity occasionally led to what was denounced as tritheism, or doctrine of three gods (so Philoponus in the 6th century). Roscellin was charged with tritheism by Anselm. Scotus Erigena taught a Neoplatonic heretical view; the mystic Richard of St Victor emphasised his view of Trinity as founded on love more than any mediæval teacher.
The Reformers clung to the doctrine as already formulated. Jakob Boehme developed a strangely mystical doctrine. Socinus and modern Unitarians are the most outspoken in their rejection of the doctrine of the Trinity, which has always been specially obnoxious to Deists and advanced Rationalists, and is little insisted on by Broad Church thinkers. Some speculative theologians and philosophers have refined away the Trinity into aspects of God's action rather than hypostases or persons as formerly understood. Clarke and the semi-Arians of the 18th century were not trinitarian in the full sense.
Tripartite divisions have had a special charm for philosophers from Plato to Hegel; to some of these the term Trinity has been applied. The Trimurti (q.v.) is called the Hindu Trinity.
See the articles ARIUS, ATHANASIUS, SOCINUS, SPIRIT (HOLY), UNITARIANISM; the handbooks of systematic theology and histories of dogma; Waterland, Bull, and Pearson of the older English systematists; and especially Donner's great works.