AGNUS DEI. - Liturgical formula introduced into the Mass by Pope Sergio I, following an ancient Greek usage, with direct allusion to the expression of Giovanni Battista (Io. 1, 29). It was to be sung with the response: Miserere nobis, repeated several times during the solemn breaking of the consecrated bread. Since this solemnity in the 11th century was reduced to the celebrant alone, the chant was limited to three invocations, the third with the response: Dona nobis pacem. In requiem Masses, however, according to a more ancient usage of the Ambrosian liturgy, the following is repeated: Dona eis requiem, with the addition of sempiternam the third time in the Roman liturgy, and et locum indulgentiae cum sanctis tuis in gloria, in the Ambrosian liturgy. The same formula of the A. D. is found already almost identically in the Gloria in excelsis (v.), a hymn which, according to the Constitutiones Apostolorum (VIII, 13, 13), was to be said in a shorter form by the people immediately before Communion. From this usage the Ecce A. D. etc. was perhaps introduced into the Roman liturgy, recited before distributing Communion to the faithful: a formula which, moreover, is found identically in the ancient liturgy of s. Giacomo. The same invocation of the A. D. is found as early as the 7th century (Ordo of s. Armando) at the conclusion of the litanies, both in the liturgy and in private devotion.
Paris 1925, p. 481; G. Destefani, La Santa Messa nella Liturgia Romana, Turin n.d., p. 743 ff. Filippo Oppenheim