The setup for this parable (from verses 21-27, paraphrased)

Jesus went to the Temple to teach. The chief priests and Jewish leaders came and challenged Him. "What right do you have to do these things? Who gave you this authority?"

Jesus told them that He'd answer them if they'd answer His question. He asked them, "The baptism of John, whence was it? from heaven or from men?"

They talked amongst themselves, knowing that if they answer "Heaven," then they're left having to explain why they didnt follow John -- and that if they answer "From men," then the people will get rowdy on them since they loved St. John the Baptist and believed what he said.

So they copped out, unable to tell their real thoughts out of fear of what people might think of them, and said, "We know not."

Jesus answered, "Neither do I tell you by what authority I do these things" -- and then He told them this parable.

Matthew 21:28-32

But what think you? A certain man had two sons; and coming to the first, he said: Son, go work today in my vineyard. And he answering, said: I will not. But afterwards, being moved with repentance, he went. And coming to the other, he said in like manner. And he answering, said: I go, Sir; and he went not.

Which of the two did the father's will? They say to him: The first. Jesus saith to them: Amen I say to you, that the publicans and the harlots shall go into the kingdom of God before you. For John came to you in the way of justice, and you did not believe him. But the publicans and the harlots believed him: but you, seeing it, did not even afterwards repent, that you might believe him.

Commentary
By St. John Chryostom

After this, He saith, "What think ye? A man had two sons; and he saith to the first, go, work to-day in the vineyard. But he answered and said, I will not: but afterward he repented, and went. And he came to the second, and said likewise. And he answered and said, I go sir: and went not. Whether then of them twain did the will of his father? They say, the first."

Again He convicts them by a parable, intimating both their unreasonable obstinacy, and the submissiveness of those who were utterly condemned by them. For these two children declare what came to pass with respect to both the Gentiles and the Jews. For the former not having undertaken to obey, neither having become hearers of the law, showed forth their obedience in their works; and the latter having said, "All that the Lord shall speak, we will do, and will hearken,"in their works were disobedient. And for this reason, let me add, that they might not think the law would benefit them, He shows that this self-same thing condemns them, like as Paul also saith," Not the hearers of the law are just before God, but the doers of the law shall be justified." For this intent, that He might make them even self-condemned, He causes the judgment to be delivered by themselves, like as He does also in the ensuing parable of the vineyard.

3. And that this might be done, He makes trial of the accusation in the person of an other. For since they were not willing to confess directly, He by a parable drives them on to what He desired.

But when, not understanding His sayings, they had delivered the judgment, He unfolds His concealed meaning after this, and saith, "Publicans and harlots go into the kingdom of Heaven before you. For John came unto you in the way of righteousness, and ye believed him not; but the publicans believed him; and ye, when ye had seen it, repented not afterwards, that ye might believe him.

For if He had said simply, harlots go before you, the word would have seemed to them to be offensive; but now, being uttered after their own judgment it appears to be not too hard.

Therefore He adds also the accusation. What then is this? "John came," He saith, "unto you," not unto them, and not this only, but; also "in the way of righteousness." "For neither with this can ye find fault, that he was some careless one, and of no profit; but both his life was irreprehensible, and his care for you great, and ye gave no heed to him."

And with this there is another charge also, that publicans gave heed; and with this, again another, that "not even after them did ye. For ye should have done so even before them, but not to do it even after them was to be deprived of all excuse;" and unspeakable was both the praise of the one, and the charge against the other. "To you he came, and ye accepted him not; he came not to them, and they receive him, and not even them did ye take for instructors."

See by how many things is shown the commendation of those, and the charge against these. To you he came, not to them. Ye believed not, this offended not them. They believed, this profited not you.

But the word, "go before you," is not as though these were following, but as having a hope, if they were willing. For nothing, so much as jealousy, rouses the grosser sort. Therefore He is ever saying, "The first shall be last, and the last first." Therefore He brought in both harlots and publicans, that they might provoke them to jealousy.

For these two indeed are chief sins, engendered of violent lust, the one of sexual desire, the other of the desire of money. And He indicates that this especially was hearing the law of God, to believe John. For it was not of grace only, that harlots entered in, but also of righteousness. For not, as continuing harlots, did they enter in, but having obeyed and believed, and having been purified and converted, so did they enter in.

Seest thou how He rendered His discourse less offensive, and more penetrating, by the parable, by His bringing in the harlots? For neither did He say at once, wherefore believed ye not John? but what was much more pricking, when, He had put forward the publicans and the harlots, then He added this, by the order of their actions convicting their unpardonable conduct, and showing that for fear of men they do all things, and for vainglory. For they did not confess Christ for fear, test they should be put out of the synagogue; and again, of John they dared not speak evil, and not even this from reverence, but for fear. All which things He convicted by His sayings, and with more severity afterwards did He go on to inflict the blow, saying, "But ye, when ye knew it, repented not afterwards, that ye might believe him."

For an evil thing it is not at the first to choose the good, but it is a heavier charge not even to be brought round. For this above all maketh many wicked, which I see to be the case with some now from extreme insensibility.

But let no one be like this; but though he be sunk down to the extremity of wickedness, let him not despair of the change for the better. For it is an easy thing to rise up out of the very abysses of wickedness.

Heard ye not how that harlot, that went beyond all in lasciviousness, outshone all in godly reverence. Not the harlot in the gospels do I mean, but the one in our generation, who came from Phoenice, that most lawless city. For she was once a harlot among us, having the first honors on the stage, and great was her name everywhere, not in our city only, but even as far as the Cilicians and Cappadocians. And many estates did she ruin, and many orphans did she overthrow; and many accused her of sorcery also, as weaving such toils not by her beauty of person only, but also by her drugs. This harlot once won even the brother of the empress, for mighty indeed was her tyranny.

But all at once, I know not how, or rather I do know well, for it was being so minded, and converting, and bringing down upon herself God's grace, she despised all those things, and having cast away the arts of the devils, mounted up to heaven.

And indeed nothing was more vile than she was, when she was on the stage; nevertheless, afterwards she outwent many in exceeding continence, and having clad herself with sackcloth, all her time she thus disciplined herself. On the account of this woman both the governor was stirred up, and soldiers armed, yet they had not strength to carry her off to the stage, nor to lead her away from the virgins that had received her.

This woman having been counted worthy of the unutterable mysteries, and having exhibited a diligence proportionate to the grace (given her) so ended her life, having washed off all through grace, and after her baptism having shown forth much self-restraint. For not even a mere sight of herself did she allow to those who were once her lovers, when they had come for this, having shut herself up, and having passed many years, as it were, in a prison. Thus "shall the last be first, and the first last;" thus do we in every case need a fervent soul, and there is nothing to hinder one from becoming great and admirable:

4. Let no man then of them that live in vice despair; let no man who lives in virtue slumber. Let neither this last be confident, for often the harlot will pass him by; nor let the other despair, for it is possible for him to pass by even the first. Hear what God saith unto Jerusalem, "I said, after she had committed all these whoredoms, Turn thou unto me, and she returned not."When we have come back unto the earnest love of God, He remembers not the former things. God is not as man, for He reproaches us not with the past, neither doth He say, Why wast thou absent so long a time? when we repent; only let us approach Him as we ought. Let us cleave to Him earnestly, and rivet our hearts to His fear.

Such things have been done not under the new covenant only, but even under the old. For what was worse than Manasseh? but he was able to appease God. What more blessed than Solomon? but when he slumbered, he fell. Or rather I can show even both things to have taken place in one, in the father of this man, for he the same person became at different times both good and bad. What more blessed than Judas? but he became a traitor. What more wretched than Matthew? but he became an evangelist. What worse than Paul? but he became an apostle. What more to be envied than Simon? but he became even himself the most wretched of all.

How many other such changes wouldest thou see, both to have taken place of old, and now taking place every day? For this reason then I say, Neither let him on the stave despair, nor let him in the church be confident. For to this last it is said, "Let him that thinketh he standeth, take heed lest he fall;"and to the other, "Shall not he that falleth arise?"and, "Lift up the hands which hang down, and the feeble knees."Again, to these He saith, "Watch;" but to those, "Awake, thou that sleepest and arise from the dead."For these need to preserve what they have, and those to become what they are not; these to preserve their health, those to be delivered from their infirmity, for they are sick; but many even of the sick become healthy, and of the healthy many by remissness grow infirm.

To the one then He saith, "Behold, thou art made whole, sin no more, lest a worse thing come unto thee;"but to these, "Wilt thou be made whole? Arise, take up thy bed, and go unto thine house."For a dreadful, dreadful palsy is sin, or rather it is not palsy only, but also somewhat else more grievous. For such a one is not only in inactivity as to good works, but also in the active doing of evil works. But nevertheless, though thou be so disposed, and be willing to rouse thyself a little, all the terrors are at an end.

Though thou hast been so "thirty and eight years," and art earnest to become whole, there is no one to hinder thee. Christ is present now also, and saith, "Take up thy bed," only be willing to rouse thyself, despair not. Hast thou no man? but thou hast God. Hast thou no one to put thee into the pool? but thou hast Him who suffers thee not to need the pool. Hast thou had no one to cast thee in there? but thou hast Him that commands thee to take up thy bed.

Thou mayest not say, "While I am coming, another steppeth down before me."For if it be thy will to go down into the fountain, there is none to hinder thee. Grace is not consumed, is not spent, it is a kind of fountain springing up constantly; by His fullness are we all healed both soul and body. Let us come unto it then even now. For Rahab also was a harlot, yet was she saved; and the thief was a murderer, yet he became a citizen of paradise; and while Judas being with his Master perished, the thief being on a cross became a disciple. Such are the wonderful works of God. Thus the magi approved themselves, thus the publican became an evangelist, thus the blasphemer an apostle.

5. Look at these things, and never despair, but be ever confident, and rouse thyself. Lay hold only on the way that leads thither, and thou wilt advance quickly. Shut not up the doors, close not up the entrance. Short is the present life, small the labor. But though it were great, not even so ought one to decline it. For if thou toil not at this most glorious toil that is spent upon repentance and virtue, in the world thou wilt assuredly toil and weary thyself in other ways. But if both in the one and the other there be labor, why do we not choose that which hath its fruit abundant, and its recompense greater.

Yet neither is this labor and that the same. For in worldly pursuits are continual perils, and losses one upon another, and the hope uncertain; great is the servility, and the expenditure alike of wealth, and of bodies, and of souls; and then the return of the fruits is far below our expectation, if perchance it should grow up.

For neither doth toil upon worldly matters everywhere bear fruit; nay but even, when it hath not failed, but has brought forth its produce even abundantly, short is the time wherein it continues.

For when thou art grown old, and hast no longer after that the feeling of enjoyment in perfection, then and not till then doth the labor bear thee its recompense. And whereas the labor was with the body in its vigor, the fruit and the enjoyment is with one grown old and languid, when time has dulled even the feeling, although if it had not dulled it, the expectation of the end suffers us not to find pleasure.

But in the other case not so, but the labor is in corruption and a dying body, but the crown in one incorruptible, and immortal, and having no end. And the labor is both first and short-lived; but the reward both subsequent and endless, that with security thou mayest take thy rest after that, looking for nothing unpleasant.

For neither mayest thou fear change any more or loss as here. What sort of good things, then, are these, which are both insecure, and short-lived, and earthly, and vanishing before they have appeared, and acquired with many toils? And what good things are equal to those, that are immovable, that grow not old, that have no toil, that even at the time of the conflicts bring thee crowns?

For he that despises money even here already receives his reward, being freed from anxiety, from rivalry, from false accusation, from plotting from envy. He that is temperate, and lives orderly, even before his departure, is crowned and lives in pleasure, being delivered from unseemliness, ridicule, dangers of accusation,and the other things that are to be feared. All the remaining parts of virtue likewise make us a return here already.

In order therefore that we may attain unto both the present and the future blessings, let us flee from vice and choose virtue. For thus shall we both enjoy delight, and obtain the crowns to come, unto which God grant we may all attain, by the grace and love towards man of our Lord Jesus Christ, to whom be glory and might forever and ever. Amen.