Explanation of the Prayers and Ceremonies of Holy Mass, Dom Prosper Guéranger - Agnus Dei

After this mingling, the Priest bowing before the Most Holy Sacrament, and joining his hands, recalls the words of Saint John Baptist and says: Agnus Dei, qui tollis peccata mundi, miserere nobis. These words could hardly be better introduced than now. Thus it is that Holy Church is wont to seek everywhere, the most lovely things in order to blend them together in one beautiful whole, in the great action of the Holy Sacrifice. Therefore does she take up the song which the angels sing in Heaven, and she too cries with them: Sanctus, Sanctus, Sanctus, Dominus Deus Sabaoth. Then she adds the gladsome shout of the Hebrew children: Benedictus qui venit in nomine Domini. But now she sings with the Precursor Agnus Dei. Yea, at this moment, the Lord is verily and indeed Himself the immolated Lamb, and twice does she implore of him, who hath taken our sins upon him, to have mercy on us, miserere nobis. The third time she adds: Dona nobis pacem, because the Eucharist is, as we have said, the Sacrament of peace, by means of which all the faithful become united together.

In Masses of the Dead, instead of miserere nobis, there is said dona eis requiem, and the third time, sempiternam is added, which very clearly expresses the character of the petition we are making for the souls of the faithful departed; we are asking for them, not now, as formerly, union in peace, but rest in the Eternal Peace.