For the kingdom of heaven is like unto a man [that is] an householder, which went out early in the morning to hire labourers into his vineyard.
Note 1 at Mt 20:1: This parable on the kingdom of heaven begins with the word "for," which is a conjunction, meaning that this parable is a further explanation of His previous statements. The teaching on the hundredfold return that precedes this parable of the laborers is concluded with, "But many that are first shall be last; and the last shall be first" (Mt 19:30). Then Jesus concluded this parable with the same statement about the last being first (Mt 20:16). Therefore, the parable that is placed between the two must be dealing with the same matter.
As mentioned in note 8 at Mr 10:30, some people could misuse this promise of the hundredfold return and believe that God owes it to them. This parable of the laborers is intended to balance this promise and teach the lesson that rewards are based on grace and not debt. "Where is boasting then? It is excluded. By what law? of works? Nay: but by the law of faith" (Ro 3:27). This faith is in God's grace (Eph 2:8).
The Scriptures teach (and this parable illustrates) that salvation is not earned but is a gift (Ro 5:17, Eph 2:8-9, and Tit 3:4-7). Just as with these laborers, some who are converted to the kingdom at the last minute and who, therefore, have virtually no good works to their credit will be just as cleansed and will enjoy the same benefits of salvation as those who have walked with the Lord in obedience for years. That's because none of us are saved because we earned it. We're saved by grace through faith (Eph 2:8-9). Therefore, we all benefit from Jesus' performance and not our own.