“Do not think that I have come to bring peace on earth; I have not come to bring peace, but a sword. For I have come to set a man against his father, and a daughter against her mother, and a daughter-in-law against her mother-in-law; and a man’s foes will be those of his own household.”
St. Hilary of Poitiers and St. Augustine offer commentaries on Matthew 10:34-36:
Mystically; A sword is the sharpest of all weapons, and thence it is the emblem of the right of authority, the impartiality of justice, the correction of offenders. The word of God, we may remember, is likened to a sword (Eph. 6:17, Heb. 4:12); so here the sword that is sent upon the earth is his preaching poured into the heart of man. The five inhabiting one house, whom he divides three against two, and two against three, we may explain thus: the three are the three parts of man, the body, the soul, and the will; for as the soul is bestowed in the body, so the will has power of using both in any way it chooses; and thence when a law is given it is given to the will. But this is only found in those who were first formed by God. By the sin and unbelief of the first parent, all the generations of men since have had sin for the father of their body, and unbelief for the mother of their soul. And as each man has his will within him, there are thus five in one house. When then we are renewed in the laver of baptism, by virtue of the word we are set apart from our original guilt, and severed, as it were, by the sword of God, from the lusts of this our father and mother, and thus there is great discord made in one house; the new man finding his foes within, he seeks with joy to live in newness of spirit; they which are derived from the old stock, lust to remain in their old pleasures.
(Quæst. in Matt. q. 3.) Otherwise; I am come to set a man against his father; for he renounces the devil, who was his son; the daughter against her mother, that is, the people of God against the city of the world, that is, the wicked society of mankind, which is spoken of in Scripture under the names of Babylon, Egypt, Sodom, and other names. The daughter-in-law against her mother-in-law, that is, the Church against the Synagogue, which according to the flesh, brought forth Christ the spouse of the Church. They are severed by the sword of the Spirit, which is the word of God. And a man’s foes are they of his household, those, that is, with whom he before lived as intimates.
Source: Thomas Aquinas. (1841). Catena Aurea: Commentary on the Four Gospels, Collected out of the Works of the Fathers: St. Matthew. (J. H. Newman, Ed.) (Vol. 1, pp. 396–397). Oxford: John Henry Parker.