Whether there should be a sermon at Mass?
1. It would seem that there ought not be a sermon at Mass. For at Mass the most holy Eucharist is consecrated, offered and received, as both Sacrament and Sacrifice; yet a sermon appertaineth neither to a sacrament, which worketh ex opere operato, nor to a sacrifice, which requireth but a priest to offer it and a victim to be offered. Therefore there ought not be a sermon at Mass.
2. Furthermore, Our Lord Himself is the Eternal Word made incarnate, and it would seem superfluous and blasphemous to supplement the Sacrament and Sacrifice of the Word with a merely human word (sermo).
3. Again, Our Lord at His last supper first consecrated the Eucharist and administered it to His disciples, and then gave His discourse by example and word afterward (cf. S. John xiii-xvii), “when supper was done” (S. John xiii, 2), and for the most part after saying, Rise, let us go hence (S. John xiv, 31), which doth answer to the dismissal at Mass, Ite missa est. Therefore the sermon should only be preached after Communion, or, better still and more evangelically, after all of Mass is ended, and, moreover, after the priest has first washed the feet of all present, and, still further, only once a year on Maundy Thursday, and only in the Upper Room in Jerusalem, following this mandate of Our Lord in all particulars.
4. Moreover, at Mass the dispositive instruction is given in the first part, when the very words of the inspired Scriptures are read, God’s own revelation to His people: whether the Old Testament is read or not as a preface to what follows, first some writing of the Apostles or their Acts is read, and finally the Gospel as the pinnacle, being the very teaching of the Lord Himself. It would therefore be wicked presumption for the priest to think to preach next, as if he were more important and his doctrine more clear than that of Christ, just as some Friars Minor think only to speak of Christ in passing ere they talk long of St Francis. Rather, therefore, if any sermon be given, it ought be before all the readings, since the celebrant is neither the Christ, nor an Apostle, nor a Prophet, but only a mere man such as were the Gentiles before they received any revelation from God at all. By extension, the sermon ought not be at Mass, but even before Mass.
To the contrary, Through many tribulations we must enter into the kingdom of God, as the Apostle says (Acts xiv, 21).
I respond, saying: many times the words of the preacher trouble his hearers, and it is in this manner that we pass through many tribulations before we enter upon the heavenly mysteries, which in this mortal life are the greatest foretaste and concealed presence of the kingdom of God. Most wisely, then, Holy Church doth permit sermons to be preached. Yet even in her practice it may be seen that she doth not favour them without limit or end. It was attested by Sozomen in the fifth century that no one preacheth in Rome, let alone priests, and we should always look to the example of the Holy Roman Church, Mother of all the churches. Moreover, of old only bishops were permitted to preach, which could not be unless preaching were not an unmitigated blessing. St Augustine, when as yet but a priest, only preached because his bishop knew not Latin sufficiently to try the patience of his people.
1. In reply to the first objection, while neither a sacrament nor a sacrifice necessarily requireth a sermon, neither absolutely excludeth being accompanied by one, if the sermon render those present more in need of the sacrament and more like unto innocent victims about to be slain. Indeed, Our Lord came not to minister to the healthy but to the sick, and so it is providential that we be rendered sick of hearing the preacher’s words ere we receive the healing remedy in the sacrament; just as for those fainting by reason of lack of food, after hearing Our Lord’s sermon, He thereupon fed them miraculously, which was a type of the Eucharist. Furthermore, we are told to offer ourselves up as a living sacrifice (Rom. xii, 1), as Christ offered Himself as a sweet savour and oblation unto God (Eph. v, 2), and to have the priest preach at one is to be made to feel a victim, which helps one unite oneself to the sacrifice about to be consummated. This the Apostle exemplified when he preached so long that Eutychus fell out of the window and died (Acts xx, 9), before the breaking of bread (verse 10). (For Eutychus is patron of those who hear sermons, as all men know.)
2. To increase our desire to receive the Word Which alone can save us, we are made to wait for this by listening in the meantime to the word (sermo) of men, just as the despairing pagans of old groaned for thousands of years, listening to the inane babble of their pretended oracles and seers, before finally hearing of the Revelation of God in Christ; and this sufficeth to meet the second objection.
3. The third objection is confuted as follows: the Last Supper occurred before the Crucifixion, whereas all Masses occur after the Crucifixion; hence Our Lord preached after Communion but before His Sacrifice on Calvary, whereas by symmetry a sermon at Mass would therefore be given after the Consecration but before the Communion. Nonetheless, as this is the practice of the dissident Armenians (for their Patriarch preached at that part of the liturgy when he visited Melbourne), the Church Catholic and universal doth do otherwise, just as she rejecteth their peculiar use of wine unmixed with water. Also, if the priest preached after Mass, no one would remain to hear him, which would frustrate the purpose of preaching as outlined in the reply to the first objection.
4. As to the fourth objection, what was said at the end of the reply to the third objection doth also hold: for if the priest preached before Mass, no one would bother to come early, even if ever they did ordinarily. This sufficeth to prove what was said in the response above.
— from the Summa Triviæ.