“The words Barber chose for the vocal parts were an ancient part of the Latin mass.  The Agnus Dei is a supplication that has used through many centuries since being added to the Latin mass by Pope Sergius (687-701). He imported it from Orthodox Christianity. In the Eucharist service, it is placed between the Lord’s Prayer and the Eucharistic prayer that precedes the consecration of the Eucharistic Elements. It can also be used as a prayer of meditation, much like the repetition of the Hail, Mary when saying a rosary.”