This Sunday is the Sixth after Epiphany – unused earlier in the year, because Easter was early, and slotted in now to fill up the number of Sundays after Pentecost. The 1962 Breviary (both the Roman and the Dominican) supplies only one short Patristic passage, formerly the seventh Lesson at Matins and now the third, as a commentary on this Sunday's Gospel; so, having located a translation, I here append the opening of that pericope, and what St Jerome wrote of this, as given for the three last lessons of Matins prior to the late pre-conciliar reforms:
A Reading of the Holy Gospel according to Matthew (13, 31-35)
At that time: Jesus spake this parable unto the multitudes: The kingdom of heaven is like to a grain of mustard seed, which a man took, and sowed in his field. And the rest.
A Homily of St. Jerome the Priest (Liber 2 Comment. in cap. 13 Matth.)
The kingdom of heaven is the proclamation of the Gospel, and that knowledge of the Scriptures, which leadeth unto life, and whereof it is said to the Jews, The kingdom of God shall be taken from you, and given to a nation bringing forth the fruits thereof. Therefore is this kingdom like to a grain of mustard seed, which a man took and sowed it in his field. By the man that sowed it in his field, many understand to be meant the Saviour, because he is the Sower that soweth in the souls of believers; others understand every man that soweth good seed in his own field, that is, in himself and in his own heart. — But thou, O Lord, have mercy upon us. R/. Thanks be to God.
Who is he that soweth, but our own mind and soul, which take the grain from preaching, and by nourishing it in the soil, cause it to sprout in the field of our own breast? The preaching of the Gospel is the least of all doctrines. He that preacheth, for his first lesson, God made man, Christ dead, and the stumbling-block of the Cross, receiveth at first but little credit. Compare such teaching as this with the doctrines of the Philosophers, with their books, their magnificent eloquence, and their rounded sentences, and thou shalt see how the grain of the Gospel, when it is sown, is the humblest of all seeds. — But thou, O Lord, have mercy upon us. R/. Thanks be to God.
But when the doctrines of men grow up, there is therein nothing piercing, nothing healthy, nothing life-giving. The plant is drooping, and delicate, and soft. There are herbs and grass whereof it may truly be said that the grass withereth and the flower fadeth. But the grain of Gospel seed, though, when it was sown, it seemed to be the least of all seeds, when once it is rooted in the soul of man, or in the whole world, groweth not into an herb, but becometh a tree: so that the birds of the air (whereby we may understand, either the souls of believers, or the angelic powers bound to the service of God,) come and lodge in the branches thereof. I consider that the branches of the Gospel tree, which groweth from the grain of mustard seed, are the divers developments of doctrine, on which the birds above mentioned find resting-places. — But thou, O Lord, have mercy upon us. R/. Thanks be to God.
What St Jerome says is most wise: the Gospel message is the least of seeds, scandal to Jews, nonsense to Greeks, and yet the wisdom and the power of God: in Bl John Henry's words, "the stately tree rears itself aloft", and whereas the doctrines of men prove but low growths and feeble, the doctrine of Christianity is strong and magnificent in its reach.