Adam Clarke on Matthew 21:28

A certain man had two sons - Under the emblem of these two sons, one of whom was a libertine, disobedient, and insolent, but who afterwards thought on his ways, and returned to his duty; and the second, a hypocrite, who promised all, and did nothing; our Lord points out, on the one hand, the tax-gatherers and sinners of all descriptions, who, convicted by the preaching of John and that of Christ, turned away from their iniquities and embraced the Gospel; and, on the other hand, the scribes, Pharisees, and self-righteous people, who, pretending a zeal for the law, would not receive the salvation of the Gospel.