A history of the feast from the good old Catholic Encyclopædia
The first Sunday after Pentecost, instituted to honour the Most Holy Trinity. In the early Church no special Office or day was assigned for the Holy Trinity. When the Arian heresy was spreading the Fathers prepared an Office with canticles, responses, a Preface, and hymns, to be recited on Sundays. In the Sacramentary of St. Gregory the Great (P.L., LXXVIII, 116) there are prayers and the Preface of the Trinity. The Micrologies (P.L., CLI, 1020), written during the pontificate of Gregory VII (Nilles, II, 460), call the Sunday after Pentecost a Dominica vacans, with no special Office, but add that in some places they recited the Office of the Holy Trinity composed by Bishop Stephen of Liège (903-20) By other the Office was said on the Sunday before Advent. Alexander II (1061-1073), not III (Nilles, 1. c.), refused a petition for a special feast on the plea, that such a feast was not customary in the Roman Church which daily honoured the Holy Trinity by the Gloria, Patri, etc., but he did not forbid the celebration where it already existed. John XXII (1316-1334) ordered the feast for the entire Church on the first Sunday after Pentecost. A new Office had been made by the Franciscan John Peckham, Canon of Lyons, later Archbishop of Canterbury (d. 1292). The feast ranked as a double of the second class but was raised to the dignity of a primary of the first class, 24 July 1911, by Pius X (Acta Ap. Sedis, III, 351). The Greeks have no special feast. Since it was after the first great Pentecost that the doctrine of the Trinity was proclaimed to the world, the feast becomingly follows that of Pentecost.
For a thousand years or so, give or take a few centuries, the old Athanasian Creed was recited in the Divine Office, laterly only at Prime on Trinity Sunday. After the late council it vanished along with the office of Prime. Along with some pretty stern admonishes about sticking to the Catholic faith, it gave a digest of what to believe about the Holy Trinity. Herewith the Athanasian Creed:
Whosoever will be saved, before all things it is necessary that he hold the Catholic Faith. Which Faith except everyone do keep whole and undefiled, without doubt he shall perish everlastingly. And the Catholic Faith is this, that we worship one God in 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, of the Son and of the Holy Ghost is all One, the Glory Equal, the Majesty Co-Eternal. Such as the Father is, such is the Son, and such is the Holy Ghost. The Father Uncreate, the Son Uncreate, and the Holy Ghost Uncreate. The Father Incomprehensible, the Son Incomprehensible, and the Holy Ghost Incomprehensible. The Father Eternal, the Son Eternal, and the Holy Ghost Eternal and yet they are not Three Eternals but One Eternal. As also there are not Three Uncreated, nor Three Incomprehensibles, but One Uncreated, and One Uncomprehensible. So likewise the Father is Almighty, the Son Almighty, and the Holy Ghost Almighty. And yet they are not Three Almighties but One Almighty.
So the Father is God, the Son is God, and the Holy Ghost is God. And yet they are not Three Gods, but One God. So likewise the Father is Lord, the Son Lord, and the Holy Ghost Lord. And yet not Three Lords but One Lord. For, like as we are compelled by the Christian verity to acknowledge every Person by Himself to be God and Lord, so are we forbidden by the Catholic Religion to say, there be Three Gods or Three Lords. The Father is made of none, neither created, nor begotten. The Son is of the Father alone; not made, nor created, but begotten. The Holy Ghost is of the Father, and of the Son neither made, nor created, nor begotten, but proceeding.
So there is One Father, not Three Fathers; one Son, not Three Sons; One Holy Ghost, not Three Holy Ghosts. And in this Trinity none is afore or after Other, None is greater or less than Another, but the whole Three Persons are Co-eternal together, and Co-equal. So that in all things, as is aforesaid, the Unity in Trinity, and the Trinity in Unity, is to be worshipped. He therefore that will be saved, must thus think of the Trinity.
Furthermore, it is necessary to everlasting Salvation, that he also believe rightly the Incarnation of our Lord Jesus Christ. For the right Faith is, that we believe and confess, that our Lord Jesus Christ, the Son of God, is God and Man.
God, of the substance of the Father, begotten before the worlds; and Man, of the substance of His mother, born into the world. Perfect God and Perfect Man, of a reasonable Soul and human Flesh subsisting. Equal to the Father as touching His Godhead, and inferior to the Father as touching His Manhood. Who, although He be God and Man, yet He is not two, but One Christ. One, not by conversion of the Godhead into Flesh, but by taking of the Manhood into God. One altogether, not by confusion of substance, but by Unity of Person. For as the reasonable soul and flesh is one Man, so God and Man is one Christ. Who suffered for our salvation, descended into Hell, rose again the third day from the dead. He ascended into Heaven, He sitteth on the right hand of the Father, God Almighty, from whence he shall come to judge the quick and the dead. At whose coming all men shall rise again with their bodies, and shall give account for their own works. And they that have done good shall go into life everlasting, and they that have done evil into everlasting fire. This is the Catholic Faith, which except a man believe faithfully and firmly, he cannot be saved.
The source for this translation is, once again, the aforesaid good old Catholic Encyclopædia
. The goCE employed the Marquess of Bute's translation.