But what think you?.... See Gill on Mat 18:12.

a certain man had two sons. This is a parable; the design of which is to show the hypocrisy and deceit of the Scribes and Pharisees, in pretending to works of righteousness, and not doing them; and to reprove them for their disbelief and rejection of John's ministry; and to make it appear, that the worst of sinners in the Jewish nation were preferable to them; and that many of them were, and would be, happy, when they would be miserable. By the "certain man", in the parable, God is designed; who, though he is not a man, nor to be represented by any human form; yet, as man is the image of God, he is therefore, in an improper and figurative sense, compared to man, and set forth by him; which may be allowed in a metaphorical and parabolical way: and though the Son of God only assumed human nature, and really became man; yet God, the Father, seems rather to be here intended, who is sometimes compared to a husbandman and a vinedresser; see Joh 15:1 and as appears from the relation of the "two sons" unto him; by whom are meant not Jews and Gentiles; for the latter can never be intended by the first son; for these were not sons in such sense as the Jews were, nor were upon an equal foot of sonship with them, as the parable supposes; much less were they called first, and bid to work in the vineyard: but, on the contrary John the Baptist, Christ, and his apostles, were first, and only sent to the Jews; and God, as yet, was not come even in the external ministry of the word to the Gentiles; nor were they brought to repentance and obedience: but by them are meant two sorts of people, among the Jews, the Scribes and Pharisees, and publicans and sinners; as the application of the parable, by our Lord himself, most clearly shows: these were both the sons of God; not only by creation, as all men are, all having, in this sense, but one common father, whose offspring they be; but also by national adoption; for to all, who were Israelites, according to the flesh, whether good men, or bad men, alike belonged the general privilege of adoption, Rom 9:4. This publicans and sinners had an equal right to, as well as the Scribes and Pharisees, though they were not all the sons of God by special grace, or spiritual adoption:

and he came to the first; the publicans and sinners among the Jews, by the ministry of John the Baptist, Christ, and his disciples, who first and chiefly preached to such sort of persons,

and said, son, go work today in my vineyard: by the "vineyard", is meant the kingdom of God, or of heaven, the Gospel church state, the then present dispensation of things, which was set up, and which men were called to embrace and enter into; the doors of which the Pharisees, who pretended to have the key of knowledge, did all they could to shut up, and hinder persons going in, as they refused to do themselves: this is called it a "vineyard"; See Gill on Mat 20:1. To work in it signifies to hear the word preached, to believe in the Messiah, embrace his doctrines, and submit to his ordinances, particularly the ordinance of baptism, which was the then principal ordinance of that dispensation. The time of working in it is "today"; directly, immediately, and whilst it is day; for the hour cometh when no man can work, and when all these means and ordinances will be at an end, and attending on them will be over: the argument used to engage hereunto, is taken from the relation the person stood in as a "son", highly favoured by God, with the blessing of national adoption, besides that of natural sonship common to all mankind.

But what think ye? A certain man had two sons; and he came to the first and said, Son, go work to-day in my vineyard--for true religion is a practical thing, a "bringing forth fruit unto God."