9 What then can we deserve, who are commanded to emulate God, and are perhaps in a way not so much as to equal the publicans? For if “to love them that love us” be the part of publicans, sinners, and heathens: when we do not even this (and we do it not, so long as we envy our brethren who are in honor), what penalty shall we not incur, commanded as we are to surpass the scribes, and taking our place below the heathens? How then shall we behold the kingdom, I pray you? How shall we set foot on that holy threshold, who are not surpassing even the publicans? For this He covertly signified, when He said, “Do not even the publicans the same?”
And this thing most especially we may admire in His teaching, that while in each instance He sets down with very great fullness the prizes of the conflicts; such as “to see God,” and “to inherit the kingdom of Heaven,” and “to become sons of God,” and “like God,” and “to obtain mercy,” and “to be comforted,” and “the great reward:” if anywhere He must needs mention things grievous, He does this in a subdued tone. Thus in the first place, the name of hell He has set down once only in so many sentences; and in some other instances too, it is with reserve that He corrects the hearer, and as though he were managing His discourse rather in the way of shaming than threatening him; where He says, “do not even the publicans the same?” and, “if the salt have lost its savor;” and, “he shall be called least in the kingdom of Heaven.”
And there are places where He puts down the sin itself by way of punishment, leaving to the hearer to infer the grievousness of the punishment: as when He says, “he has committed adultery with her in his heart;” and, “he that puts away causes her to commit adultery;” and, “That which is more than these is of the evil one.” For to them that have understanding, instead of the mention of the punishment, the very greatness of the sin is sufficient for correction.
Wherefore also He here brings forward the heathens and the publicans, by the quality of the person putting the disciple to shame. Which Paul too did, saying, “Sorrow not, even as the rest which have no hope;” and, “Even as the Gentiles which know not God.”
And to signify that He requires nothing very overpowering, but a little more than was accustomed, He says,
“Do not even the Gentiles the same?” Yet nevertheless He stops not the discourse at this, but makes it end with His rewards, and those good hopes, saying,
“Be therefore perfect, as your Heavenly Father.”
And He intersperses everywhere abundantly the name of the heavens, by the very place thoroughly elevating their minds. For as yet, I know not how, they were somewhat weak and dull.
Source: Homilies on the Gospel of St. Matthew (New Advent)