John Chrysostom

But when, not understanding His sayings, they had delivered the judgment, He unfolds His concealed meaning after this, and saith, "Publicans and harlots go into the kingdom of Heaven before you. For John came unto you in the way of righteousness, and ye believed him not; but the publicans believed him; and ye, when ye had seen it, repented not afterwards, that ye might believe him." For if He had said simply, harlots go before you, the word would have seemed to them to be offensive; but now, being uttered after their own judgment it …

Homily on the Gospel of Matthew 67 ↗

Jerome

It should be known that in the correct copies it is read not The last, but The first, that they might be condemned by their own sentence. But should we prefer to read, as some have it, The last, the explanation is obvious, to say that the Jews understood the truth, but dissembled, and would not say what they thought; just as though they knew that the baptism of John was from heaven, they would not say so.

Catena Aurea by Aquinas ↗

Hilary of Poitiers

The first son represents the people who are from the Pharisees. Urgently admonished by God through the prophecy of John to conform themselves to his commandments, they remained insolent, disobedient and contemptuous to God's warnings. They put their faith in the law and despised repentance from sin, glorying instead in the noble prerogative that they had from Abraham. …

Commentary on Matthew 21.13-14 ↗

Thomas Aquinas

Then he draws out the verdict: "which of the two did the will of his father?" The first did not promise, but he did it; the second promised, but did not do it. "Which of these did the will of his father?" They answer and say to him: "the first," because "it is better not to vow, than after a vow not to pay what was promised," Eccl 5:4. And 2 Pet 2:21: "It is better not to know the way of truth, than after knowing it to turn back;" for therein is a double sin: the sin of disobedience and the transgression of a vow. …

Commentary on Matthew ↗