The fundamental truth on which everything in the Christian religion rests, is the dogma of the Holy Trinity from whom all comes (Epistle), and to whom all baptized in His name must return (Gospel).
In the course of the cycle, having called to our minds in order, God the Father, Author of creation, God the Son, Author of redemption, and God the Holy Ghost, Author of our sanctification, the Church today, before all else, recapitulates the great mystery by which we acknowledge and adore the Unity of Nature and Trinity of Persons in almighty God (Collect).
The dogma of the Holy Trinity is affirmed, in the liturgy, on every hand. It is in the Name of the Father and of the Son and of the Holy Ghost that we begin and end the Mass and Divine Office, and that we confer the Sacraments. All the Psalms end with the Gloria, the Hymns with the Doxology, and the Prayers by a conclusion in honor of the three Divine Persons. Twice during the Mass we are reminded that it is to the Holy Trinity that the Mass is being offered.
A votive Mass in honor of the Holy Trinity was composed in the seventh century, and in the eleventh century the Abbey of Cluny established the custom of using it for a feast of the Holy Trinity on this Sunday. Bishop Stephen of Liege composed its office in the tenth century. The feast was officially adopted by the Roman liturgy in 1334 at Avignon by Pope John XXII and was later promoted in rank by St. Pius X.
That we may ever be armed against all adversity, let us today, with the liturgy, make our solemn profession of faith in the Holy and Eternal Trinity and His indivisible Unity.