There are two parts to this answer. First of all, the Agnus Dei (Greek for ‘Lamb of God’ – one of the many titles ascribed by Scripture to Jesus) is part of what is known as the Ordinary of the Mass. “O Lamb of God, who takes away the sin of the world, have mercy upon us. O Lamb of God, who takes away the sin of the world, have mercy upon us. O Lamb of God, who takes away the sin of the world, grant us your peace.” Sung or said after the Prayer of Blessing over the bread and wine, the Ordinary are texts which are used in all of the church’s Eucharistic (Communion) services and give a sense of familiarity and shape to our common worship.
Many of the great Classical, Renaissance, and Baroque composers typically held positions in the church and so they would write Mass settings — instrumental and sung compositions — of these Ordinary texts. Because we are blessed in our 10am Sunday service with a talented choir and our very own Choir Director and Composer John Butler, we usually follow the classical form of the Mass at this service, and the choir sings a setting of Agnus Dei written by John.
But here is the second part of that answer. In 1945, Anglican theologian Gregory Dix wrote the highly influential book The Shape of the Liturgy. In it, he argued that it was not so much the words of the liturgy but its “shape” which mattered. His study of the liturgy’s historical development led him to formulate what he called the Four Action Shape of the Liturgy: Offertory, Consecration, Fraction, and Communion. (source: Wikipedia).
His work and research has allowed a freer sense in our Anglican tradition of what constitutes Common Prayer and to develop Anglican worship into a balance of the familiar and the fresh. The Agnus Dei, in modern Anglican thought, is no longer considered an essential part of our worship together, but rather, an option which can help us in shaping the prayer and Communion which has been passed down to us from the very first followers of Jesus. As such, it is rarely used in the more spare and simple mid-week Eucharists we celebrate here at St. George’s, and it is important to know that it can be interchangeable with other Scriptural texts or hymns (or even silence) which can also help us transition from the Blessing
and Breaking (Consecration and Fraction) and into our time of Communion. For various suggestions for words, anthems or hymns to be used, including the Agnus Dei, check out pages 226-228 of our Book of Alternative Services.