From The Gospel of Matthew, by Curtis Mitch and Edward Sri, commenting on Matthew 21:28-32
Despite the temple authorities’ evasive answer, Jesus exposes their rejection of John the Baptist with a parable about a man with two sons. The father asks his sons to work in the vineyard, an Old Testament image for Israel that Jesus has already utilized (20:1–16; Isa 5:1–7). The first son refuses his father. In a culture where sons are to honor and obey their fathers (Sir 3:1–16), the son’s initial I will not is a shameful act of defiance. But he later changed his mind and went out into the field.
Although the second son agreed to work the field and even honorably addressed his father as “lord” (translated “sir” in the NAB), in the end he disobeyed and did not do the father’s will—reminiscent of Jesus’ teaching in 7:21 about those who call him “Lord” but do not do the Father’s will and do not enter the kingdom.
Obviously, the first son is the one who did his father’s will. Even the chief priests and elders recognize that. But what Jesus says next would have utterly dumbfounded them. Jesus tells them that the tax collectors and prostitutes will enter the kingdom of God before you. Tax collectors and prostitutes were considered to be at the bottom of the socioreligious scale and outside God’s covenant—the kind of people the chief priests and elders looked down on the most. Yet, like the first son, these notorious sinners, who rebelled initially, repented when they heard the exhortation of John the Baptist. That Jesus would say these sinful outsiders will enter God’s kingdom before the chief priests and elders would have been completely astounding—and offensive.
At the same time, Jesus links the chief priests and elders with the second son. They had the law, and by taking office they affirmed that they would do God’s will. But when God sent his prophet John the Baptist calling all to repent, they did not believe him. They will find themselves watching the sinners enter God’s kingdom before them. It is implied that if they fail to repent, they will be left out of the kingdom (8:11–12).