« TE DEUM ». — Also called the Hymnus ambrosianus or Hymnus S. Trinitatis, it is a solemn hymn of praise and thanksgiving in honor of the Most Holy Trinity. It consists of two parts with an addition of eight psalm-like verses. The first part (« Te Deum... confietur Ecclesia »: 1-10; « Patrem... Spiritum »: 11-13) comprises a praise of the Father followed by a kind of Trinitarian doxology. The second (« Tu rex... numerari »: 14-21) is a Christological praise. The eight added verses were detached from the « Gloria in excelsis », where they were originally united, and derive from: vv. 22 and 23 from Psalm 27:9; vv. 24 and 25 from Psalm 144:2 (of the Roman version); V. 26 is a morning verse, which also recurs in the canonical hour of « Prima »; V. 27 derives from Psalm 122:3; V. 28 from Psalm 32:22 and V. 29 from Psalm 30:2.
The Te Deum does not seem to be an original composition; hence it is more properly spoken of as a compiler rather than an author. In fact, the first part is the oldest and derives from an anaphora: it includes the biblical Trisagion in the liturgical style, cites the Prophets (of the early Christian centuries after the Apostles, mentioning only martyrs, not confessors); vv. 7-9 seem to come from Cyprian (De mortalitate, ch. 26). An ancient Greek version of the first part (vv. 1-10) exists. The second part, with its Christological praise, corresponds well to the 4th century and the beginning of the 5th, as the rules of the « cursus » observed there (more than in the first part) indicate the time of composition in the 4th-5th century. The psalm-like verses point to a time before 389, the era of St. Jerome’s revision.
Recently, in comparison with a group of Greek evening hymns, Baumstark has indicated a very ancient type of hymns, among which the Te Deum might fittingly be included. A very charming legend attributes its composition to SS. Ambrose and Augustine; but this dates only from the 12th century. SS. Hilary, a Sisibutus, and an Abbondius are also mentioned as authors. In the oldest manuscripts the Te Deum is antepigraphic. With valid arguments, Morin proves that its author is Nicetas of Remesiana ([v.] d. after 414); though not without contradictions. The Te Deum was known in the 5th century in the monastic island of Lérins, from where SS. Caesarius (in 489-98 at Lérins) and Aurelian of Arles prescribe it in their Rules for Sundays. From Lérins, where it was in use around 415-20, St. Patrick may have brought the Te Deum to Ireland on his return in 432; indeed, the Irish manuscripts are the oldest that preserve its text.
Il T. D. si usa dal tempo di s. Benedetto (Reg., c. XI) alla fine dell'Ufficio notturno delle domeniche e delle feste (a Roma, in S. Pietro al tempo di Amalario, secc. VIII-IX), soltanto nelle feste anniversarie dei papi, nei giorni quando alla Messa si dice il « Gloria ». Nella liturgia celtica il T. D. serve come canto eucaristico « quando comunicant sacerdotes »; lo stesso nella liturgia gallicana, ove si trova anche come canto di ringraziamento. Nella liturgia romana entra come canto di lode, di ringraziamento, di trionfo (nella consacrazione dei vescovi, nella benedizione degli abati, re e regine e in tutti i momenti più solenni).
Da notare alcune varianti importanti del testo antico: al V. 16 « ad liberandum mundum suscepisti hominem » (oggi « ad liberandum suscepturus hominem ») e il V. 21 « gloria munerari » (oggi « in gloria numerari »).
Nella melodia si distinguono due incisi, uno dopo il V. 13 e l'altro dopo il V. 21. La melodia della prima parte (1-13) sembra cantarsi sul tipo del Prefazio, la seconda invece al modo salmodico IV gregoriano.