Despite assertions to the contrary, the Mass formulary and Office for the feast of Corpus Christi were almost certainly composed by St Thomas Aquinas (1225-1274) at the behest of Pope Urban IV. (Dr Donald Prudlo gives an outstanding overview of the origins of the feast here.) That includes the Collect for the feast:
Deus, qui nobis sub Sacramento mirábili passiónis tuae memoriam reliquisti: tríbue, quáesumus, ita nos Córporis et Sánguinis tui sacra mysteria venerári, ut redemptiónis tuae fructum in nobis júgiter sentiámus: Qui vivis et regnas.
Which I translate as:
O God, who in this wonderful Sacrament hast left us a memorial of Thy passion: grant us, we beseech, to venerate the sacred mysteries of Thy body and blood in such a way that we may ever sense within us the fruit of Thy redemption. Who livest and reignest.
The prayer, which is inspired by Biblical passages such as Luke 22, 19 and 1 Corinthians 11, 24-26 that describe Jesus instituting the Eucharist during the Last Supper, is noteworthy as an early example of an oration addressed to Jesus Christ, a practice that has been the subject of recent discussion (see here and here).
Further, the Collect is a useful reminder of various Catholic doctrines concerning the Eucharist.
First, the Blessed Sacrament--and the Mass that confects it--is not a reenactment of the Last Supper but a re-presencing of the suffering, death, resurrection, and ascension of Jesus Christ. As the Epistle reading for the feast reminds us, “For as often as you shall eat this bread, and drink the chalice, you shall shew the death of the Lord, until He come.” (1 Cor. 11, 26)
Second, by mentioning the sacred mysteries of Christ’s body and blood during a feast dedicated only to the Lord’s body, the Collect indirectly calls our attention to the doctrine of concomitance, the belief that even the smallest particle of the Host contains not only the whole of Christ’s body, but also His blood, soul, and divinity as well. The Resurrection of Jesus Christ from the dead means that what was separated on Good Friday (blood, body, and soul) has been reunited. And His Ascension into Heaven, where He sits in glory at the right hand of the Father, means that the reunion shall last forever: never again is Christ’s blood to be separate from His body, not even in the different species of His sacrament. As Aquinas writes in his magnificent sequence for the Mass, Lauda Sion Salvatorem.
Caro cibus, sanguis potus:Manet tamen Christus totusSub utráque specie.
A sumente non concísus,Non confractus, non divísus:Integer accípitur.
Which I translate as:
Flesh as food, blood as drink,Yet the whole Christ remainsUnder either kind.
By being consumedHe is not cut up, broken, or divided:He is received in His entirety.
In other words--and this brings us to our third point--the Blessed Sacrament is truly wonderful. Our current bodies are marvelous things, a source of artistic and scientific amazement, but St Paul tells us that they are a mere acorn in comparison to the tree that will be our glorified bodies, the kind that Jesus Christ currently has thanks to the Resurrection. His risen body is an object of far greater wonder because it defies the laws of space, time, and matter as we know them. It is that marvelous risen body that we receive in the Eucharist, and it is one of the reasons why we will never fully understand this sacrament. In the words of the Sequence:
Quod non capis, quod non vides,Animósa firmat fides,Praeter rerum órdinem.
Which I translate as:
What you do not grasp, what you do not see,An ardent Faith confirmsOutside the order of things.
Finally, the petition asks that veneration of the Eucharist will lead not simply to the fruit of redemption but to our feeling this fruit. Salvation, friendship with God, and theosis (which I take to be among the fruits of redemption) are great and wonderful things, but the only thing better than having them is knowing that you have them and rejoicing in them. Indeed, we may view the feast of Corpus Christi as an attempt to make this wish come true. By exciting our love for the Lord in His Blessed Sacrament, the feast aspires to fill us with a holy joy. For as we sing in the Sequence about this feast:
Sit laus plena, sit sonóra,Sit jucunda, sit decóraMentis jubilatio.
Which I translate as:
Let our praise be full, let it resound,Let our heart's jubilationBe sweet and charming.