Free gift of salvation
The direct object of this parable seems to be, to show that though the Jews were first called into the vineyard, at length the gospel should be preached to the Gentiles, and they should be admitted to equal privileges and advantages with the Jews. The parable may also be applied more generally, and shows, 1. That God is debtor to no man. 2. That many who begin last, and promise little in religion, sometimes, by the blessing of God, arrive at a great deal of knowledge, grace, and usefulness. 3. That the recompense of reward will be given to the saints, but not according to the time of their conversion. It describes the state of the visible church, and explains the declaration that the last shall be first, and the first last, in its various references. Till we are hired into the service of God, we are standing all the day idle: a sinful state, though a state of drudgery to Satan, may be called a state of idleness. The market-place is the world, and from that we are called by the gospel. Come, come from this market-place. Work for God will not admit of trifling. A man may go idle to hell, but he that will go to heaven, must be diligent.
There is great proneness in us to think that we have too little, and others too much of the tokens of God's favour; and that we do too much, and others too little in the work of God. But if God gives grace to others, it is kindness to them, and no injustice to us. See here the nature of envy. It is an evil eye, which is displeased at the good of others, and desires their hurt. It is a grief to ourselves, displeasing to God, and hurtful to our neighbours: it is a sin that has neither pleasure, profit, nor honour. Let us forego every proud claim, and seek for salvation as a free gift. Let us never envy or grudge, but rejoice and praise God for his mercy to others as well as to ourselves.
-- extract from Matthew Henry's Concise Commentary