I like the text of the hymn, and how the piece could be split into a Trinitarian form with the refrain recurring 3 times interspersed with 3 hymn verses. On the negative side, I would just be repeating myself, the six verse syllabic hymn felt a little boring and the polyphony is nothing special. I don't like doing polyphony just to say we did it, I like when it has that inimitable sound that nothing else on earth has. I have to assume there is some better music out there for one of the deep mysteries of the Christian faith, no? Unfortunately I'm not an expert in this area of repertoire and need some ideas.

Thank you @AnimaVocis. I have looked through that category before, and did find one piece I liked the look of, Sulcius' Gloria Tibi Trinitas: https://www.cpdl.org/wiki/images/3/32/Sulcius,_Gloria_tibi.pdf.

Edit: I misread your post - I have not in fact looked through that entire category! Will do.

What does your choir sing on Trinity Sunday?

I've also found a dearth of worthwhile music for Trinity. We have done the Tchaikovsky as Richard has already linked, and Samuel Webbe Sr's Benedicimus Deum coeli. Last year we did Rutter's Cantate Domino, but that divides into eight parts.

The proximity of the solemnity to Eastertide, Ascension and Pentecost doesn't help with allowing time to prepare something more interesting.

EDIT: Just found that Lassus has a 4-voice Tibi laus. Recording here.

I've also found a dearth of worthwhile music for Trinity.

There are things out there. Maybe they need to be edited to be generally useful. Hmm, who might do that?

Last year we sang the following:

O sacred Feast (Healey Willan)
https://youtu.be/5Wqe-1V90XU?si=WrN7QGCVnordXcJU

What's reputed to be Palestrina's most beautiful motet is in fact a stunning five-voice, two-part piece for the Trinity. I've done it many times over the years, and it's so worth revisiting. Always ravishingly beautiful. (We sing it in the one-step-down transposition.)

These Isaac settings,
Benedicta sit sancta (Heinrich Isaac)
Alleluia: Benedictus es, Domine (Heinrich Isaac)
Benedicimus Deum caeli (Heinrich Isaac)

and this from Tomás Luis de Victoria, https://www.cpdl.org/wiki/images/b/b6/TresSunt.pdf

@Polyfollia That's really nice. Baroque is not the style at my parish, but I would be happy to know that someone somewhere in the universal Church is doing it.

Today published on CPDL.org, not 3, not 4 but 8 part 'high carate' polyphony by Dominique Phinot: "Sancta Trinitas"!

https://www.cpdl.org/wiki/index.php/Sancta_Trinitas_(Dominique_Phinot)

Many thanks @ MIck Swithinbank!