Agnus Dei Qui tollis peccata mundi. The use of this modified form of part of the Gloria in Excelsis (q. v.), founded on John, i. 29, seems to be referred to in the rubric for Easter Eve in the Sacramentary of St. Gelasius, A.D. 492. In the time of Pope Sergius I. [687-701] it was ordered by him to be sung at the Communion of priest and people…Anastatius Bibliothecarius records this in Historia de Vitis Bomanorum Pontificum. It is the opinion of Bona that Pope Sergius ordered it to be sung thrice; Le Brun, on the contrary, thinks it was only sung once. In the 11th century the last clause of its third repetition, "miserere nobis," began to appear as "dona nobis pacem” and a little later in Masses for the dead, the last clause, instead of "dona nobis pacem,” runs as a special prayer for the departed, "dona cis requiem sempiternam." This occurs also in the English Missals of Sarum, York and Hereford, and is the universal custom of the Roman Church at the present day, which also repeats the words, "Ecce Agnus Dei, ecce Qui tollis peccata mundi,” as the priest turns to deliver the sacramental wafer to the people.
According to the Sarum Use the Agnus Dei was incorporated in the Litany, but only to be sung twice, and the third clause is placed first….
The Agnus Dei has also come into English use through the German, in the following manner:—
(i.) 0 Lamm Gottea unschuldig. By Nicolaus Decius, or Hovesch, first published in Low German in the Geystlyke leder, Rostock, 1531, and in High German in V. Schumann's Gesang-Buch, Leipzig, 1539. It has been much used in Germany at Holy Communion during the distribution of the elements; on Good Friday, at the close of sermon; and on other occasions.
The translations in common use are:—
1. 0 Lamb of God most holy. By A. T. Russell as No. 26 in the Dalston German Hospital Collection, 1848, in 2 stanzas of 7 lines, repeated in his own Psalms & Hymns, 1851, No. 156, in 3 st. In both cases the stanzas are identical, save in line 7.
2. 0 Lamb of God, most stainless. By Miss Winkworth, as No. 46 in her Chorale Book for England, 1863, in 3 st., identical, save in line 7.
3. 0 Lamb of God, most Holy. Once for us sinners dying. By Miss Borthwick, in full from Knapp, contributed as No. 66 to Dr. Pagenstecher's Collection, 1864.
4. Lamb of God, without blemish! No. 75, in the Ohio Lutheran Hymnal, 1880, in 3 st., identical, save line 7. [Rev. James Mearns, M.A.]
--Excerpts from John Julian, Dictionary of Hymnology (1907)