What I find most striking about the liturgical texts for Trinity Sunday is the emphasis on worship. We find it in the Collect (even as mangled in the 'reforms') used in the Roman and Anglican usages, and in the Preface (before it was truncated for Anglicans by Cranmer); come to think about it, this is the point of the doxology (Glory be to ..."). And for some of us there is the Quicuncue vult, the Athanasian Creed which was not written by S Athanasius (in the Pius XII form of the Roman Rite, this is said at Prime only on this Sunday of the year?). The point about the Trinity Sunday is not how Three can be One, but that we worship Father, Son, and Spirit; we worship the Trinity in Unity and the Unity in Trinity. Possessors of the Breviary will not need to be told about the insistence of its antiphons upon Doxology: giving glory to the blessed Trinity and the undivided Unity.
For earlier generations, Trinity Sunday was the commonest day for ordination. It was for S John Henry Newman. From his ordination as an Anglican to the diaconate to his ordination in Rome as a Roman Catholic, ordination, for Newman, meant Trinitytide. And how appropriate this was. On Pentecost Sunday, we celebrated the outpouring by God the Father through his Son of the Holy Spirit; through those glorious days of Octave we Alleluiad the Holy Spirit and prayed daily in the Sequence and the Office Hymns for the Holy Spirit to "come" upon us. And on Trinity Sunday, Veni Creator Spiritus was sung over us ... in my case, it was in Christ Church Cathedral just along the road from here ... as the climax of this Octave; the bishop laid his hands upon us "for the Office and Work of a Priest in the Church of God, now committed unto thee by the imposition of our hands". As in the ancient Western Pontificals, the imposition of hands was accompanied by the paschal commission of the Lord himself: "Receive the Holy Spirit; whose sins thou dost remit, they are remitted ...". Sadly, these dominical words have diappeared, with much else, from the post-Conciliar Pontificale Romanum.
I find it impossible to hear Veni Creator Spiritus without memories crowding the tears to the back of my eyes; and there is another detail of the day's liturgy which remains powerfully with me; I wonder if it did with Newman. From early in his Anglican days, he learned to love the Roman Breviary; and he will have known that on Trinity Sunday, the first two readings of Mattins were the passage from Isaiah 6 about the Glory filling the Temple of the Lord at Jerusalem, and all the Seraphim singing Holy Holy Holy. You will remember that it ends with the seraph bringing a burning coal from the altar and touching the prophet's mouth; and "I heard the voice of the Lord saying 'Whom shall I send, and who will go for us?' Then I said 'Here am I, send me'". From 1871, also this passage featured in the Prayer Book for Mattins on Trinity Sunday; how sad that the Liturgia Horarum knew so much better than to continue this usage.
Priests are given many job-descriptions, because there are many different modes in which priesthood is exercised. But in all of them, the heart of the purpose of priesthood is to give Glory to the blessed and undivided Trinity; to offer to the Father the glorious and adorable Sacrifice of his Son's Body and Blood "in the unity of the Holy Spirit", because the Holy Sacrifice of the Mass is the Act of glorification of the Trinity; whatever else a priest has to do or does, it comes second to, or is derived from, the duty of standing day by day at an altar and joining the angels and archangels and all the company of Heaven and lauding and magnifying his holy Name, evermore praising him and saying: Holy, Holy, Lord God of Hosts.