I am glad to have the opportunity to join the discussion here at the RLI. My name is Aaron Cummings and I am an interested lay person. Thanks much.
In most traditional liturgies which have survived to this day, the Agnus Dei is said immediately after Consecration of the element, and the declaration: “The Peace of the Lord be with you always.” In some churches, the minister goes so far as to say while elevating the host: “Behold the Lamb of God. Behold Him that taketh away the sins of the world.” The words of the Agnus Dei are:
“O Lamb of God which taketh away the sins of the world, have mercy upon us.
O Lamb of God which taketh away the sins of the world, have mercy upon us.
O Lamb of God which taketh away the sins of the world, grant us thy peace.”
These words are nice enough as it is. They are well-worth singing in the service, for our Lord is the Lamb of God who does take away the sins of the world. We should beg his mercy and peace.
What follows is merely a suggestion and I would welcome any feedback.
1) These words coming at this point in the service suggest that now Christ is here in a special way in the newly consecrated elements, here, that is, in such a way that we could pray to the elements.
2) If you quote John in pointing to the elements as Christ, then are you more pointedly asking the congregants to pray to the elements?
3) Would these words work better as a hymn of confession at the start of service? Consider that male lambs (rams in particular) were sacrificed as the trespass offering, the first of the sacrifices in the Levitical service. The Trespass was the sacrifice for the heavy, high-handed sins (Lev. 5:14-6:7). That Christ is our “lamb” specially identifies him specifically as the trespass offering, for when offering the other sacrifices (burnt, grain, peace, and sin), the sacrificing saint could offer goats, bulls, doves, pigeons and even bread. Christ certainly fulfills the other sacrifices, but John didn’t say, “Behold the goat of God”, or “Behold the bread of God”. He said, “Behold the Lamb of God,” highlighting the trespass sacrifice. The Agnus Dei seems to work very well as a hymn of confession rather than a hymn of joy at the Lord’s table. What about praying the Agnus and then the Kyrie as sister hymns of confession?
Reformed Liturgical Resources 
Call me an incurable conservative, but I am always a little skittish about schemes to move around the chants that comprise the Ordinary.
First of all, there is a beautiful symmetry built into the Ordinary: Kyrie-Gloria-Credo-Sanctus-Agnus Dei. In the center of this “arch” is the confession of the Church’s faith. The Creed is the keystone: without it the entire structure crumbles. On either side of the Creed is a song of the Angels, one from the Old Testament and one from the New: the Gloria in excelsis (Luke 2) and the Sanctus (Isaiah 6). At the beginning and the end are prayers for mercy, one in Greek and one in Latin: the Kyrie eleison and the Agnus Dei.
Second, although Reformed Protestant theology, on the whole, does not allow for the idea that Christ is present “in a special way” in the newly consecrated elements, the Reformed confessions agree that in the action of eating the bread and drinking the wine, believers do commune with Christ “in a special way.” Calvin, in his commentary on John 6, is especially insistent that, in the Supper, we feed upon the flesh of Christ in a way that we do not at any other time. So while Reformed theology may not recognize a local presence, it nevertheless recognizes a Real Presence. For this reason, I believe it is most appropriate to sing the Agnus Dei right before eatng the bread, in anticipation of the sacred action in which we are about to participate.
I feel similarly about those who, in recent years in some Reformed circles, have made the “discovery” that we should move the Sursum Corda to the beginning of the service, instead of at the beginning of the Eucharistic prayer. I think that all this innovation does is to render that liturgy idiosyncratic and, frankly, clunky and awkward. Calvin connected the Sursum Corda specifically to the Supper, not to the entire worship service.
Good thoughts from both of you! This highlights why this blog exists. Liturgical theology is complex and has a long history. We Americans are so quick to re-invent the wheel, or to reject wheels altogether. The important thing is for pastors and sessions to really get into these issues and not simply cut-and-paste from other churches.
I’ll try to avoid responding here to the issue of adherence to traditional forms. (Perhaps a new thread would be appropriate.) I’ll instead speak to the agnus points.Let me say, however, that I favor the ancient over being blown about by every wind of liturgical change. If a change is to be made, it must, in my opinion, be due to Biblical necessity, not human whim.
1) Concerning liturgical symetry. First, this certainly isn’t a Biblical requirement (“Thou shalt remember the liturgical chiasm and keep it holy.”), although sensitivity to balance and form is absolutely requisite to the art of liturgy. Second, Bible truth trumps human art. In other words, if Bible study requires that we change our service, then out with chiasms. Third, if we wanted to retain the chiasm, here’s a suggestion
2 Prayers of Mercy:
Gloria
Credo
Sanctus
2 Prayers of Thanksgiving (Perhaps Psalm 100–THE prayer of thanksgiving–and another)
2) Concerning Real Presence. Amen. Christ is there among us “in a special way” (not the most felicitous phrase, for Christ says, “whereever two or more are gathered…” We should say, “He’s just there.”) Christ is known to us in the eating as Calvin and Cranmer were at pains to state. Anything we sing as we dine with Christ should certainly be given great thought. Perhaps asking for mercy is approprate. This leads me to two open ended points for another time: A) Christ is also known to in the hearing of his word. We should not divorce the service of the word from the service of the table. In fact, it is an error to think of them as separate services. B) Confession belongs at the beginning of the service. Once we have confessed, we are clean and so may enter God’s presence ritually.
There’s a lot here. I’ll stop before I promise to many new threads.
An interesting point I just learned in the course of reading The Study of Liturgy (eds. Jones, Wainwright, Yarnold): “The Agnus Dei introduced a quite alien note into the Roman liturgy. Indeed it was an importation from an Eastern source by Pope Sergius (687-701), who himself had an ancestry of Sicilian and Syrian background. In the Roman rite it was sung at the time of the Fraction,” (pg. 234).
I’m not quite sure what the author means by a “quite alien note,” and I don’t present this as a conclusive piece of evidence for the question of where to place the Agnus Dei. But, as we discuss these issues, we do need to know about where the different elements of the liturgy came from. I tend to think “importations” from the East can be a positive thing. Just as we don’t want to absolutize our Presbyterian “traditions,” nor do we want to priviledge the Roman tradition. Truly recovering our catholic heritage would seem to mandate taking the best of the Eastern liturgies as well.