This parable certainly refers to the Lord's call to live a life of faith and of service in God's vineyard. Each calling takes place at different times and in different ways, but extending it to the whole of human history, its significance is all the more charged and powerful.
We do not know when the dawn of the Gospel parable occurred, but we can guess that at successive intervals throughout the day of creation, men were given the opportunity to hear the Lord's voice inviting them to serve life and to live it in a bright, gracious, harmonious and spiritually evolved manner so as to glorify the splendor of creation. The invitation to work honorably in the vineyard of creation was also linked to a salary, a reward that could only be the currency of full, shared well-being, the fullness of peace and happiness. How many times have men evolved according to their capacities and intelligence, only to start over again because they lost their sense of existence, their spiritual axis, and their inner harmony and ability to see God's countenance?
We cannot know how often and in what ways the Lord of life has gone out onto the roads of life and history to call His children and inspire, guide, enlighten, and awaken them. We can only guess how each different civilization and historical period responded, more or less in tune, to the Lord's calling to serve life and to enjoy its fruits of love and wisdom.
Throughout this long process of understanding and evolution, the jarring note is the grumbling of the first "laborers in the vineyard" about the Lord of life's behavior with respect to the wage agreed upon. It denotes that, although they were the first to go into the vineyard they did so for nothing if there was still envy in their hearts for the Lord of life's goodness. Being the first was futile if they still thought ill of the Lord of life, thought ill of His covenant, His justice, His omnipotent vision of life and of all things.
The first went out into the vineyard for nothing, as in their hearts the sense of duty had not evolved into passionate initiative, free dedication, if the burden of living or the non-sense did not evolve into full gratitude and gratuitousness.
At any time in history in which you are called to serve life and to enjoy it, make sure never to grumble against the Lord of life.