1911 Encyclopædia Britannica/Agnus Dei
AGNUS DEI, the figure of a lamb bearing a cross, symbolical of the Saviour as the "Lamb of God." The device is common in ecclesiastical art, but the name is especially given in the Church of Rome to a small cake made of the wax of the Easter candles and impressed with this figure. Since the 9th century it has been customary for the popes to bless these cakes, and distribute them on the Sunday after Easter among the faithful, by whom they are highly prized as having the power to avert evil. In modern times the distribution has been limited to persons of distinction, and is made by the pope on his accession and every seven years thereafter.
Agnus Dei is also the popular name for the anthem beginning with these words, which is said to have been introduced into the missal by Pope Sergius I. (687-701). Based upon John i. 29, the Latin form is Agnus Dei, qui tollis peccata mundi, miserere nobis. In the celebration of the mass it is repeated three times before the communion, and it is also appended to many of the litanies. By the judgment in the case of "Read and others v. The Bishop of Lincoln" it was decided in 1890 that the singing of the Agnus Dei in English by the choir during the administration of the Holy Communion, provided that the reception of the elements be not delayed till its conclusion, is not illegal in the Church of England.
For the various ceremonies in the blessing of the Agnus Dei see A. Vacant, Dict. de théologie (cols. 605-613).