Matthew 21:28-32 - 28 “What do you think? There was a man who had two sons. He went to the first and said, ‘Son, go and work today in the vineyard.’ 29 “‘I will not,’ he answered, but later he changed his mind and went. 30 “Then the father went to the other son and said the same thing. He answered, ‘I will, sir,’ but he did not go. 31 “Which of the two did what his father wanted?” “The first,” they answered. Jesus said to them, “Truly I tell you, the tax collectors and the prostitutes are entering the kingdom of God ahead of you. 32 For John came to you to show you the way of righteousness, and you did not believe him, but the tax collectors and the prostitutes did. And even after you saw this, you did not repent and believe him. - NIV
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. - Matthew Henry's Concise Commentary
Two sons - By those two sons our Lord intends to represent the conduct of the Jews, and that of the publicans and sinners. In my vineyard - See the notes at Matthew 21:33. To work in the vineyard here represents the work which God requires man to do.
I will not - This had been the language of the publicans and wicked men. They refused at first, and did not "profess" to be willing to go.
Repented - Changed his mind. Afterward, at the preaching of John and Christ, the publicans - the wicked - repented and obeyed.
The second ...said, I go sir; and went not - This represented the conduct of the scribes and Pharisees - "professing" to obey God, observing the external rites of religion, but opposed really to the kingdom of God, and about to put his Son to death.
Whether of them twain ... - Which of the two. "They say unto him, The first." This answer was correct; but it is strange that they did not perceive that it condemned themselves. - Barnes' Notes on the Bible