The Scripture excerpts in Latin are from the Vulgate; those in English are from the RSV Second Catholic Edition.
Here's how it appears in the 2002 Latin Missale Romanum:
132. Sacerdos genuflectit, accipit hostiam, eamque aliquantulum elevatam super patenam vel super calicem tenens, versus ad populum, clara voce dicit:The following is from the 1975 GIRM (US translation):
Ecce Agnus Dei,
ecce qui tollit peccáta mundi.
Beáti qui ad cenam Agni vocáti sunt.
Et una cum populo semel subdit:
Dómine, non sum dignus,
ut intres sub téctum meum,
sed tantum dic verbo,
et sanábitur ánima mea.
115. [T]he priest genuflects, takes the eucharistic bread, and, holding it slightly above the paten while facing the people, says: "This is the Lamb of God." With the people he adds, once only: "Lord, I am not worthy to receive you."This is reflected in the 1985 US translation of the Missal (the "Sacramentary"):
132. The priest genuflects. Taking the host, he raises it slightly above the paten and, facing the people, says aloud:The 2000 GIRM reflects a change, permitting the Host to be held over the Chalice:
This is the Lamb of God,
who takes away the sins of the world.
Happy are those who are called to his supper.
He adds, once only, with the people:
Lord, I am not worthy
to receive you,
but only say the word,
and I shall be healed.
157. When the prayer is concluded, the priest genuflects, takes the host consecrated in the same Mass, and, holding it slightly raised above the paten or above the chalice, while facing the people, says, Ecce Agnus Dei (This is the Lamb of God). With the people he adds, Domine, non sum dignus (Lord, I am not worthy).Here's how the Latin is rendered in the upcoming (2010?) English translation of the Missal:
132. The Priest genuflects, takes the host and, holding it slightly raised above the paten or above the chalice, while facing the people, says aloud:You'll notice some changes in the English translation; the underlying Latin has not changed. The new translation is superior to the old in a number of ways: 1) the words are almost identical to Scripture; 2) the words are a more accurate translation of the Latin (ecce = "behold", not "this is"; anima = "soul", not "I"); and 3) the word choice is less secular (beati as "blessed" rather than "happy").
Behold the Lamb of God,
behold him who takes away the sins of the world.
Blessed are those called to the supper of the Lamb.
And together with the people he adds once:
Lord, I am not worthy
that you should enter under my roof,
but only say the word,
and my soul shall be healed.
Also note that, in the present translation, the priest does not say "Happy are we who are called to this supper." There is great significance in saying "those" (instead of "we") and "his" (instead of "this"). First, saying "we" is over-inclusive: it presumes that everyone present at Mass is going to receive (which might not be the case). Second, saying "we" and "this" is over-exclusive: it emphasizes the local congregation and celebration, neglecting the universal character of Holy Communion, whereby all those who receive Holy Communion are partaking of the same "marriage supper of the Lamb". It's not just this supper, it's his supper, the Lamb's supper. Instead of being focused on the local present moment, our attention should be drawn to the heavenly eternal moment.
The words of the priest come from John 1:29 (the words of John the Baptist) and Revelation 19:9 (the words of an angel).
John 1:29The words said in response come from Matthew 8:8 (the words of the centurion).
Altera die videt Iesum venientem ad se et ait: "Ecce agnus Dei, qui tollit peccatum mundi."
The next day he saw Jesus coming toward him, and said, "Behold, the Lamb of God, who takes away the sin of the world!"
Et dicit mihi: "Scribe: Beati, qui ad cenam nuptiarum Agni vocati sunt!".
And the angel said to me, "Write this: Blessed are those who are invited to the marriage supper of the Lamb."
Matthew 8:8As you can see, the words have been only slightly altered. This happens from time to time in Scriptural texts which are used in the liturgy: a word is added, removed, or changed, to fit the context.
Et respondens centurio ait: "Domine, non sum dignus, ut intres sub tectum meum, sed tantum dic verbo, et sanabitur puer meus."
But the centurion answered him, "Lord, I am not worthy to have you come under my roof; but only say the word, and my servant will be healed."
The rubrics (the text in red) are very clear here. First, the priest genuflects. Then, he holds up either the Host over the paten or the Host over the Chalice. Then, while facing the people, he speaks his words aloud. We respond along with the priest.
History, Context, and Symbolism
You'll notice the rubrics makes a few points clear: 1) the priest genuflects first; 2) the priest is holding the Host in his hand over a paten or the Chalice, and is not simply holding up a paten with the Host on it; 3) the priest faces the people at this part of the Mass; 4) the priest speaks aloud; 5) the priest and people respond together; 6) our response is said once; and 7) there is no mention striking the breast during the response.
We will look at each of these in turn, pointing out some differences between the Ordinary Form ("O.F.", the Mass as revised in 1970) and the Extraordinary Form ("E.F.", the Mass as revised in 1962). In the E.F., the Communion of the priest was separate from the Communion of the people:
The priest stands on the same side of the altar as the people, and he is facing the altar (just as the people are) for most of the Mass. First, the priest genuflects and adores the Host; as he rises, he says a prayer in a low voice. Next, he takes the Host and paten in his left hand and prays the Domine non sum dignus three times (speaking the first four words out loud, and the rest in a low voice), striking his breast with his right hand once each time. Then he makes the sign of the cross over himself with the Host, says a private prayer, and consumes the Host.In the O.F., the Communion rites of the priest and people are combined and simplified. The priest makes one genuflection before the Blessed Sacrament, combining the three of the older liturgy (which were for the Host and the Chalice during his own Communion, and for the Host before distributing Communion to the faithful). This genuflection is an act of adoration for the Blessed Sacrament.
After a moment of silent meditation, he says another prayer in a low voice, uncovering the Chalice which has a pall over it (a board covered in cloth to prevent things from falling into it, still used in the O.F.), and genuflecting again. He collects any fragments of the Host from the corporal (the white cloth on which the paten, Host, and Chalice are placed on the altar, still used in the O.F.) and paten and brushes them into the Chalice. Then, holding the paten in his left hand and the Chalice in his right, he says another private prayer, makes the sign of the cross with the Chalice, and drinks from it.
Then he genuflects once more to the Blessed Sacrament to be distributed to the faithful, and turns to face them, holding a Host in his left hand over the ciborium, and he says out loud the Ecce Agnus Dei, to which the faithful respond with the Domine non sum dignus three times, striking their breast once each time.
The priest holds the Host over the paten or over the Chalice; he does not simply hold up the paten with the Host on it, nor does he hold the Host and the Chalice apart from each other (as he does at the end of the Eucharistic Prayer). The priest is showing us the Lamb, so that we can "behold" him.
The priest faces the people during this part of the Mass. Contrary to popular opinion, the O.F. assumes that the priest will be saying Mass ad orientem, that is, "facing the east", which has been the long-standing custom of Christian worship going back even to the first century. While it is permitted for the priest to stand on the other side of the altar (and thus face the people and the altar at the same time), the default posture of the Mass is for the priest to be on the same side of the altar as the people, and it contains instructions for the priest to "face the altar" at some times and to "face the people" at others. Since the preparations of the priest just prior to this part of the Mass are private (that is, they are for him and not for us), he is facing the altar during them. Once he has genuflected and taken up the Host and paten or Chalice, then he faces the faithful.
He says the Ecce Agnus Dei aloud so that the people can hear him. Again, the private preparations of the priest are said in a "low voice", since they are not being spoken to us. Since these words are being spoken to us, they are said audibly to us.
Both the priest and the people respond with the Domine non sum dignus together. Strictly speaking, the faithful did not need to say this response in the E.F., although they were certainly encouraged to do so. In the O.F., our vocal participation at this point is expected. This is also a simplification of the rite in the E.F., where the priest would say the Domine non sum dignus for himself prior to his own Communion, and then again prior to the Communion of the people.
The response, Domine non sum dignus, is said only once, which is a simplification of the three-fold response found in the E.F.
There is also no mention of the gesture of striking the breast during the response. Striking the breast is a sign of penitence, which is appropriate given the response, in which we admit that we are "not worthy". This was found in the E.F., but has been dropped from the O.F.