Somehow, the landowner is God, and the laborers he called early in the day are the Jews. They are the first and the chosen ones. When God set out in the marketplace, He saw them first, just as God saw Abraham first from a multitude of people.
The next ones are probably the early Jewish converts to Christianity. 12NN signifies a changing of AM to PM, a change from Old to New, from Judaism to Christianity.
Then the Christianization of the Roman Empire happened and it spread throughout the whole of Europe and the world through the Age of Exploration. The last ones, at 5pm, are the last converts who heard the Word before the end of days.
So in this Gospel, God went to the marketplace (to hire prophets), to till the vineyard (world). From start (dawn) to end (evening), He worked to cultivate the world in His word.
Wages are represented as "salvation". Salvation is equal for all. Whether, you are first or last, it is the same for everyone. But of course, those who came in first had to toil harder (till the land, burdened by heat) etc which represents the placing of the foundation for the vineyard (world). Those who came in mid and last are the finishing touches to produce the best possible harvest.
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