Trinity (Gr. τριάς, Lat. trinitas), a term of Christian theology denoting the coexistence in the Godhead of three persons, distinguished from each other as the Father, the Son, and the Holy Ghost or Holy Spirit. The doctrine of the Trinity is held by the Roman Catholic church, and by most of the Protestant and eastern churches. The doctrine, it is contended, is contained in all its constitutive elements in the Scriptures, and was gradually drawn up into a systematic statement as the necessity occurred of preserving or vindicating it in its integrity and purity. Supplementary to the ecclesiastical form of the dogma itself are certain theological explanations, throwing on it a fuller light, derived from the teachings of early councils, the writings of the great church fathers, or the accepted scientific language of the schools. These regard the mode of origination of the second and third persons, the relations existing between the persons in the Trinity, and their distinctive characteristics and appellations. "While the word Trinity is not to be found in the Bible, and while no passage can be adduced from the Old Testament in which the doctrine of the Trinity or its equivalent is distinctly and explicitly formulated, many texts have been quoted even by the earliest Christian writers which point to the existence of some form of plurality in the Godhead. These texts, however, being susceptible of various interpretations, are not produced as proving peremptorily the doctrine of a Trinity, but as foreshadowing the clear and distinct revelation believed to have been made in the New Testament. From it two large classes of texts are quoted as arguments for establishing the doctrine: those in which Father, Son, and Holy Spirit are mentioned in connection, and those in which these three subjects are mentioned separately, and in which their nature and mutual relation are more particularly described.

The disputes about the tri~ personality of the Godhead date from the apostolic age, and were occasioned chiefly by the prevalence of the Hellenistic and Gnostic the-osophies. Theophilus, bishop of Antioch in the 2d century, used the word τριάς, and its equivalent trinitas was first employed by Ter-tullian in the 3d century. During the ante-Kiccne period there was uninterrupted controversy about this doctrine, principally in the East, and many opinions were proscribed by the church as heretical. Among them were those of the Ebionites, who regarded Jesus as a mere man; of the Sabellians, according to whom the Father, the Son, and the Holy Ghost were only the different forms in which the one God reveals himself to men; of the Arians, who taught that the Son was not coeternal with the Father, but created by him before the world, and therefore subordinate and inferior to the Father; and of the Macedonians, who denied the personality of the Holy Ghost. The doctrine of the church was fixed by the councils of Nice (325) and Constantinople (381), which declared that the Son and Spirit are coequal with the Father in the divine unity, the Son' eternally begotten by the Father, and the Spirit proceeding from the Father. The synod of Toledo (589) declared that the Holy Ghost proceeded also from the Son (filioque), and this addition was finally adopted throughout the Latin church; but the Greeks, though at first acquiescent and silent, at length protested against this change of the creed as an innovation, and the phrase filioque still remains one of the chief hindrances of a reunion between the Greek and Roman Catholic churches. . The symbolic books of the Lutheran and Reformed churches retained the Roman Catholic doctrine of the Trinity unchanged; but it has been attacked ever since the 16th century, as contrary to both the Bible and sound reason, by a large number of theologians and by several new denominations, as the Socinians, the German theoso-phists (Weigel, Boehm, &c), the Unitarians, and the Universalists. Swedenborg referred the Trinity to the person of Christ, teaching a trinity, not of persons, but of the person, by which he understood that that which is divine in the nature of Christ is the Father, that the divine which is united to the human is the Son, and the divine which proceeds from him is the Holy Spirit. The spread of rationalism in the Lutheran and Reformed churches undermined for some time the belief in the Trinity among a large number of German theologians.

Kant held that Father, Son, and Spirit designate only three fundamental qualities in the Deity, power, wisdom, and love, or three agencies of God, creation, preservation, and government. Hegel and Schelling attempted to give to the doctrine of the Trinity a speculative basis; and after their example the modern dogmatic theology of Germany has in general undertaken a defence of the doctrine of the Trinity on speculative as well as theological grounds. Some supranaturalist theologians do not hold the strict doctrine of ecclesiastical orthodoxy, as defined by the councils of Nice and Constantinople, and the view of Sabellius especially has found in modern times many advocates. - Exhaustive works on the history of the doctrine of the Trinity have been published by Baur (Die christhelie Lehre von der DreieinigJceit, Tubingen, 3 vols., 1841-'3) and Meier (Die Lelire von der Trinitat in his-torischer Entwickelung, Hamburg, 1844). See also Hodge, "Systematic Theology " (3 vols., New York, 1872-'3).