"'But what do you think? A man had two sons, and he came to the first and said, "Son, go, work today in my vineyard." He answered and said, "I will not," but afterwards he regretted it and went. Then he came to the second and said likewise. And he answered and said, "I go, sir," but he did not go. Which of the two did the will of his father?' They said to Him, 'The first.' Jesus said to them, 'Assuredly, I say to you that tax collectors and harlots enter the kingdom of God before you. For John came to you in the way of righteousness, and you did not believe him; but tax collectors and harlots believed him; and when you saw it, you did not afterward relent and believe him'" (Matt. 21:28-32).
"But what do you think?" - Jesus prefaces the parable with this question, perhaps to alert His listeners to pay close attention to His words since they will have to make a response.
"Son, go, work today in my vineyard" - This was the request made by the father to both of his sons individually. Certainly any father has the right to speak to his children in this manner and expect them to labor for him faithfully.
The first son was rebellious and stubborn. His defiant attitude is not praiseworthy, but at least he was not a hypocrite (like his brother). He later regretted his disobedience and did go to work in the vineyard. The second son spoke respectfully and gave his word, but he did not keep it. Either this son deliberately lied or he (for whatever reason) failed to act on his good intentions.
Let it be understood by all that mere words are never enough. One must actually do what God says in order to be obedient to Him (cf. Matt. 7:21; Mark 7:6; James 1:22).
"Which of the two did the will of his father?" - Here is the question Jesus posed to His listeners. They answered correctly in stating that the "first" son did the father's will. Of course, technically speaking, neither son obeyed as he should have. The second son should have kept his word and the first should not have rebelled to begin with. But, the first son, in the end, did do the father's will. This is what is most important to God (i.e., what we become, not what we have been).
"Assuredly, I say to you that tax collectors and harlots enter the kingdom of God before you" - Jesus has been speaking specifically to the religious leaders (cf. 21:27). They had correctly answered this question (without realizing they were condemning themselves), and now Jesus applies the parable to them.
The "worst" among the common Jews (i.e., the tax collectors and harlots) are represented by the first son, and the second son depicts the "best" of the Jews (i.e., the religious leaders). Most tax collectors and harlots did not pretend to excel religiously; they openly rebelled against God's will ("I will not" - 21:29). On the other hand, the religious leaders (for the most part) professed to be very zealous in obeying God ("I go, sir" - 21:30). The reason why the tax collectors and harlots would enter the kingdom before these leaders is because they acknowledged a need for help and were willing to change. There was still hope for the religious leaders if only they would awaken to their desperate condition and repent!
"For John came to you in the way of righteousness, and you did not believe him" - John came as a Jew and a prophet of the strictest type. His righteousness was evident to all and there could not have been a prophet more acceptable to the religious leaders (cf. 11:11,18), yet they still rejected him.
"But tax collectors and harlots believed him; and when you saw it, you did not afterward relent and believe him" (cf. Luke 7:29,30). Many common Jews, like the tax collectors and harlots, turned from their rebellion at the preaching of John the baptizer (cf. Matt. 3:5,6). They regretted their sins and started doing the will of God. The religious leaders, on the other hand, witnessed the preaching of John and the repentance of many, but they refused to change. They kept hypocritically pretending to be God's servants, yet they refused to heed John's message. Often those who seem farthest away from God will genuinely embrace Him more quickly than those who are pretending to be close to Him.
Jesus aimed this parable at the religious leaders because they had refused to answer His question in Matthew 21:25 (a question that any of the common people could have answered). The religious leaders considered themselves faithful to God and much better than the lower-class Jews they despised. However, God did not think of them in such a favorable way. They promised faithful service to God but did not deliver it. They were hypocrites and needed to repent.