He answered and said, I will not: but afterward he repented, and went.

People's New Testament

21:28-31 A certain man had two sons. The two sons represent the priests, elders and scribes on the one hand, and the publicans and harlots, the sinners, on the other (Mt 21:31). Both classes were bidden to work in the Lord's vineyard. The publicans and sinners had refused, but repented at the preaching of John. The others professed to obey, but did not. The design of the parable is to show that the publicans and harlots, whom they so much despised, were morally superior to his questioners.

Scofield Reference Notes

Margin repented

Cf., Lk 15:20, the other perfect illustration of repentance. See Scofield Note: "Acts 17:30".

Jamieson-Fausset-Brown Bible Commentary

29. He answered and said, I will not-Trench notices the rudeness of this answer, and the total absence of any attempt to excuse such disobedience, both characteristic; representing careless, reckless sinners resisting God to His face.

Matthew Henry's Concise Commentary

21:28-32 Parables which give reproof, speak plainly to the offenders, and judge them out of their own mouths. The parable of the two sons sent to work in the vineyard, is to show that those who knew not John's baptism to be of God, were shamed by those who knew it, and owned it. The whole human race are like children whom the Lord has brought up, but they have rebelled against him, only some are more plausible in their disobedience than others. And it often happens, that the daring rebel is brought to repentance and becomes the Lord's servant, while the formalist grows hardened in pride and enmity.