O Lux beata Trinitas

You can read about this hymn attributed to Saint Ambrose, O Lux Beata Trinitas, at the Hymns and Carols of Christmas website.

O Lux beata Trinitas

O Lux beata Trinitas,
et principalis Unitas,
iam sol recedit igneus,
infunde lumen cordibus.

Te mane laudum carmine,
te deprecemur vespere:
te nostra supplex gloria
per cuncta laudet saecula.

Deo Patri sit gloria,
eiusque soli Filio,
cum Spiritu Paraclito,
et nunc, et in perpetuum.

(Saint Ambrose)