DEO GRATIAS. - A formula of greeting and thanksgiving already used by St. Paul (I Cor., 15, 52; II Cor., 2, 14). St. Augustine speaks of it as a distinctive word for the Christians of Africa, or as a greeting, and gives an explanation of it (Enarr. in Ps., n. 6): "Fratres, cum vident homines, D. g. dicunt... Num enim non est locus gratulationis, quando se invicem vident qui habitant in Christo?".
In the liturgical cult the faithful say this formula in response to the Benedicamus Domino; in the Roman and Ambrosian Mass after the Ite Missa est, in the Gallican and Mozarabic liturgy after the announcement of the title of a Lesson, and in the Divine Office after all the Lessons, in ancient times also after the reading of the Acts of the Martyrs, in order to thank the Lord for the teaching received from the divine word or for the incitement to do good works. The celebrant says it on Holy Saturday after the announcement of the Easter canticle of the Alleluia. The bishop says it at the presentation of the candidates for the diaconate and priesthood, after having assured himself of their worthiness, and at the consecration of bishops and abbots, when these have made the prescribed oath of fidelity. The supreme pontiff in ancient times said it on the day of the Resurrection, after having heard the number of the newly baptized (Ordo Romanus I, 3), etc.
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Filippo Oppenheim