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 (To Autolycus 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 (On Pudicity 21). 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, 986).
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 of 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 trinity as a mystery
The Vatican Council has explained the meaning to be attributed to the term mystery in theology. It lays down that a mystery is a truth which we are not merely incapable of discovering apart from Divine Revelation, but which, even when revealed, remains "hidden by the veil of faith and enveloped, so to speak, by a kind of darkness" (Constitution, "De fide. cath.", iv). In other words, our understanding of it remains only partial, even after we have accepted it as part of the Divine message. Through analogies and types we can form a representative concept expressive of what is revealed, but we cannot attain that fuller knowledge which supposes that the various elements of the concept are clearly grasped and their reciprocal compatibility manifest. As regards the vindication of a mystery, the office of the natural reason is solely to show that it contains no intrinsic impossibility, that any objection urged against it on Reason. "Expressions such as these are undoubtedly the score that it violates the laws of thought is invalid. More than this it cannot do.
The Vatican Council further defined that the Christian Faith contains mysteries strictly so called (can. 4). All theologians admit that the doctrine of the Trinity is of the number of these. Indeed, of all revealed truths this is the most impenetrable to reason. Hence, to declare this to be no mystery would be a virtual denial of the canon in question. Moreover, our Lord's words, Matthew 11:27, "No one knoweth the Son, but the Father," seem to declare expressly that the plurality of Persons in the Godhead is a truth entirely beyond the scope of any created intellect. The Fathers supply many passages in which the incomprehensibility of the Divine Nature is affirmed. St. Jerome says, in a well-known phrase: "The true profession of the mystery of the Trinity is to own that we do not comprehend it" (De mysterio Trinitatus recta confessio est ignoratio scientiae — "Proem ad 1. xviii in Isai."). The controversy with the Eunomians, who declared that the Divine Essence was fully expressed in the absolutely simple notion of "the Innascible" (agennetos), and that this was fully comprehensible by the human mind, led many of the Greek Fathers to insist on the incomprehensibility of the Divine Nature, more especially in regard to the internal processions. St. Basil, Against Eunomius I.14; St. Cyril of Jerusalem, Catechetical Lectures VI; St. John Damascene, Of the Orthodox Faith I.2, etc.).
At a later date, however, some famous names are to be found defending a contrary opinion. Anselm ("Monol.", 64), Abelard ("ln Ep. ad Rom."), Hugo of St. Victor ("De sacram." III, xi), and Richard of St. Victor ("De Trin.", III, v) all declare that it is possible to assign peremptory reasons why God should be both One and Three. In explanation of this it should be noted that at that period the relation of philosophy to revealed doctrine was but obscurely understood. Only after the Aristotelean system had obtained recognition from theologians was this question thoroughly treated. In the intellectual ferment of the time Abelard initiated a Rationalistic tendency: not merely did he claim a knowledge of the Trinity for the pagan philosophers, but his own Trinitarian doctrine was practically Sabellian. Anselm's error was due not to Rationalism, but to too wide an application of the Augustinian principle "Crede ut intelligas". Hugh and Richard of St. Victor were, however, certainly influenced by Abelard's teaching. Raymond Lully's (1235-1315) errors in this regard were even more extreme. They were expressly condemned by Gregory XI in 1376. In the nineteenth century the influence of the prevailing Rationalism manifested itself in several Catholic writers. Frohschammer and Günther both asserted that the dogma of the Trinity was capable of proof. Pius IX reprobated their opinions on more than one occasion (Denzinger, 1655 sq., 1666 sq., 1709 sq.), and it was to guard against this tendency that the Vatican Council issued the decrees to which reference has been made. A somewhat similar, though less aggravated, error on the part of Rosmini was condemned, 14 December, 1887 (Denz., 1915).