Part VII: Quam oblationem. and Consecration.

"Quam oblationem tu, Deus, in omnibus, quaesumus, benedictam, adscriptam, ratam, rationabilem, acceptabilemque facere digneris: ut nobis Corpus, et Sanguis fiat dilectissimi Filii tui Domini nostri Iesu Christi."

"Vouchsafe, O God, we beseech thee, in all things to make this oblation blessed, approved and accepted, a perfect and worthy offering: that it may become for us the Body and Blood of thy dearly beloved Son, our Lord Jesus Christ."

We ask the father to bless and accept the offering that Christ will soon make on the altar, that of his own body and blood, for all the ends of intentions which we have prayed for. Now, comes the moment. The moment upon which all heaven, all earth, all creation has waited. The king of kings, the lord of lords comes to dwell among his servants, in order to make possible the promise he made to dwell in us, and we in him. ( John 6:56-59) In so holy a moment, in which Christ himself mystically speaks his own words, those he gave at the last supper, it is not odd that many pious people bow low during these words as the missal instructs the priest.

"Qui pridie quam pateretur, accepit panem in sanctas ac venerabiles manus suas, et elevatis oculis in coelum ad te Deum Patrem suum omnipotentem tibi gratias agens, benedixit, fregit, diditque discipulis suis, dicens:
Accipite, et manducate ex hoc omnes, Hoc Est Enim Corpus Meum, Quod Pro Vobis Tradetur."

"Who the day before he suffered,( He takes the bread.) took bread into his holy and venerable hands, and with eye lifted up to heaven unto thee, God, his almighty Father, giving thanks to thee,He blessed, brake and gave it to his disciples, saying:
( He bows down)
Take, and eat ye all of this: For This Is My Body, Which Will Be Given Up For You."

He elevates the host and bells are rung by the servers, the host is censed, and many people look up and recite St. Thomas words as a statement of belief in Christ resurrection, his divinity, and his Eucharistic presence: "My Lord and my God!" (John 20:28) After elevating the host for the people's devotion, he genuflects. Originally, the genuflection at the consecration was singular, and did not occur until the consecration of the chalice. But there was in the tenth century , a heretical priest who denied Christ's physical presence in the blessed sacrament at the consecration, saying he did not become present until after the consecration of the wine. After he was denounced as a heretic, it became widespread to genuflect right after the elevation of the host as a sign of our belief that Jesus becomes present at the words of consecration.
After rising, the priest continues:

"Simili modo postquam coenatum est, accipiens et hunc praeclarum Calicem in sanctas ac venerabiles manus suas: item tibi gratias agens, benedixit deditque discipulis suis, dicens: Accepite, et bibite ex eo omnes:Accipite et bibite ex hoc omnes:

Hix Est Enim Calix Sanguinis Mei, Novi et Aeterni Testamenti. Qui Pro Vobis et Pro Multis Effundetur, In remissioem Peccatorem. Hoc Facite in Meum Commemorationem."

"Likewise after supper (He takes the chalice into is hands) taking also this goodly chalice into His holy and venerable hands, again giving thanks to Thee He blessed, and gave it to His disciples, saying: Take and drink ye all of it:

For This Is the Chalice of My Blood, The Blood of the New and Eternal Covenant. It Shall be Shed for You and for The Many, for the Remission of Sins. This Do in Remembrance of Me."

He elevates the chalice of Christ's blood for the adoration of the faithful. It's a pious custom to recite at this moment, a paraphrase of another verse from holy scripture: "Thou hast redeemed us O Lord, not with perishable things, but with thine own precious blood." (I Peter 1:18-19)
Rising to his feet, the priest sings "Mysterium Fidei", "The Mystery of faith". The mystery of faith is Christ's death on the cross, made present at the consecration. These words, Mysterium Fidei serve themselves as a proclamation that what has just occurred on the altar is the Mystery of Faith.The mystery of his presence in the blessed sacrament, which is why we sing:

"Salvator mundi, salva nos, qui per crucem et resurrectionem tuam liberasti nos."
"Savior of the World, save us! By your cross and resurrection you have set us free."