32 “Say ye not, that there are yet four months, and then comes harvest?” He was aglow for the work, and was arranging to send forth laborers. You count four months to the harvest; I show you another harvest, white and ready. Behold, I say unto you, “Lift up your eyes, and see that the fields are already white for the harvest.” Therefore He is going to send forth the reapers. “For in this is the saying true, that one reaps, another sows: that both he that sows and he that reaps may rejoice together.
I have sent you to reap that on which you have not labored: others have labored, and you are entered into their labor.” What then? He sent reapers; sent He not the sowers? Whither the reapers? Where others labored already. For where labor had already been bestowed, surely there had been sowing; and what had been sown had now become ripe, and required the sickle and the threshing. Whither, then, were the reapers to be sent? Where the prophets had already preached before; for they were the sowers.
For had they not been the sowers, whence had this come to the woman, “I know that Messias will come”? That woman was now ripened fruit, and the harvest fields were white, and sought the sickle. “I sent you,” then. Whither? “To reap what you have not sown: others sowed, and you are entered into their labors.” Who labored? Abraham, Isaac, and Jacob. Read their labors; in all their labors there is a prophecy of Christ, and for that reason they were sowers. Moses, and all the other patriarchs, and all the prophets, how much they suffered in that cold season when they sowed!
Therefore was the harvest now ready in Judea. Justly was the grain there said to be as it were ripe, when so many thousands of men brought the price of their goods, and, laying them at the apostles' feet, having eased their shoulders of this worldly baggage, began to follow the Lord Christ. Verily the harvest was ripe. What was made of it? Of that harvest a few grains were thrown out, and sowed the whole world; and another harvest is rising which is to be reaped in the end of the world.
Of that harvest it is said, “They that sow in tears shall reap with joy.” But to that harvest not apostles, but angels, shall be sent forth. “The reapers,” says He, “are the angels.” That harvest, then, is growing among tares, and is awaiting to be purged in the end of the world. But that harvest to which the disciples were sent first, where the prophets labored, was already ripe. But yet, brethren, observe what was said: “may rejoice together, both he that sows and he that reaps.” They had dissimilar labors in time, but the rejoicing they shall enjoy alike equally; they shall receive for their wages together eternal life.
Source: Tractates on the Gospel of John (New Advent)