« TE DECET LAUS ». - A hymn, or rather a brief Trinitarian acclamation in the form of a doxology.
Very ancient, it already occurs in the Apostolic Constitutions (bk. VII, ch. 48,3, with a revised variant, « Patrem per Filium in Spiritu Sancto »), and still today in the Orthros of the Byzantine liturgy (Orologion, Rome 1937, pp. 129-30). Following the example of the Eastern monks, the hymn, translated from Greek, was prescribed by s. Benedetto (Reg., ch. XI) on Sundays and feasts at the Night Office after the reading of the Gospel; in the monastic rite the doxology follows the chanting of the « Te Deum », just as in the Byzantine rite it follows the similar « Gloria in excelsis », in order to conclude the Night Office. In the Middle Ages it was also used in the Office of the secular clergy in place of the « Te Deum » when another Office was recited at night or in the morning (Double Office; Radulfo de Rivo, De canon. observatione, prop. 21 in fine, ed. C. Mohlberg, II, Münster 1915, pp. 123, 21). A doxology similar to the T. d. l. was already used in the synagogue service on Saturday morning.