From the Breviary of St. Pius V, 1568, a reading from the fourth Mystagogical Catechesis of St. Cyril, bishop of Jerusalem and Doctor of the Church, appointed for the Octave Day of Corpus Christi.
The teaching of blessed Paul seems of itself amply sufficient to make certain your faith concerning the Divine Mysteries; and you, having been made worthy thereof, have become, so to speak, of one Body and of one Blood with Christ. For he proclaimed that on the night He was betrayed, our Lord Jesus Christ took bread, and when He had given thanks, He broke it, and gave it to His disciples, saying: Take, and eat, this is my Body. And taking the cup, and giving thanks, He said: Take this, and drink; this is my Blood. Since therefore He Himself has proclaimed this and said, “This is my Body”, who will dare henceforth to doubt that it is so? And since He again has said so insistently “This is my Blood”, who would ever doubt, and say that it is not his Blood?When St. Thomas Aquinas composed the office of Corpus Christi, known from its first antiphon as Sacerdos in aeternum, he wrote not only the musical texts such as the antiphons and hymns, but also the sermon to be read in the first and second nocturns of Matins, according to the custom of his times. This sermon, Immensa divinae largitatis beneficia, is found in almost all pre-Tridentine breviaries; however, it is not long enough to provide readings for Matins on each day of the octave. Other readings had therefore to be selected for the remaining days; the bull Transiturus by which Pope Urban IV promulgated the feast was a popular choice. The 1529 Breviary of the Roman Curia has readings from another bull, Si Dominum in sanctis of Clement V, confirming that of Pope Urban; at Bamberg, Eugenius IV's Excellentissimi Corporis, granting or confirming a long list of indulgences attached to the observation of Corpus Christi, was read as the sixth lesson of Matins on the feast itself. (This single reading occupies almost 140 lines!) The Sarum Breviary gives several very long tracts from the Decree of Gratian, broadly speaking the medieval code of Canon Law; the third part, called On Consecration, is a long florilegium of texts from the Church Fathers and various other sources, and is quite suitable for spiritual reading despite being essentially a law textbook.
Once, at Cana in Galilee, He turned water into wine, which has a certain similarity to blood; and shall we think him too little worthy of our belief, when He said He would turn wine into Blood? Being called to that marriage, by which two bodies are joined, He did this miracle, which none expected. Shall we not all the more firmly believe that He has given us His Body and Blood, to be our food and drink, and thus receive them with all certainty as His Body and his Blood? For under the appearance of bread He gives us His Body, and under the appearance of wine, His Blood, so that when you shall receive it, you may taste the Body and Blood of Christ, being made a partaker of the same Body and Blood. Thus indeed do we become Christ-bearers, that is, bearing Christ in our bodies, when we receive His Body and Blood into our members; thus, according to the blessed Peter, do we come to share in the divine nature.
…Wherefore I would not have you understand these things, as if they were merely and simply bread, merely and simply wine; for they are the Body and Blood of Christ. For even if your senses deny this fact, yet let faith confirm you in this belief. Judge not the thing by the taste thereof, but let faith assure thee beyond all doubt, that you have been made worthy to partake of the Body and Blood of Christ.
When the Roman Breviary was revised after the Council of Trent, almost all of these passages were substituted with readings from the Fathers of the Church. The sermon of St. Thomas remains on the feast itself and the following day; the remaining days of the octave are given over to Saints John Chrysostom, Cyprian and Ambrose, and last of all, on the octave itself, the passage quoted above from St. Cyril of Jerusalem. It is not difficult to see in the choice of such readings a response by the Catholic Church to the early Protestants, and their rejection of the traditional doctrine of the Eucharist. Papal bulls or medieval canon law collections would hold no authority with the “reformers” of the age (Martin Luther burned both at Wittenberg), whether openly Protestant or uncertain Catholics, who were many in that age. The writings of the Fathers, on the other hand, were frequently appealed to as proof that the teachings of the Protestants were in fact those of the primitive Church, and things like Eucharistic processions and Adoration later corruptions of the Medieval era. In such a climate, the writings of St. Cyril in particular were a source of profoundest embarrassment to early and later Protestant controversialists. By including such a passage in a corpus of sermons that begins with a work of St. Thomas Aquinas, the Breviary of St. Pius V asserts a continuity of doctrine from St. Paul to the Church Fathers to the greatest theologian of the medieval, scholastic tradition.