John Chrysostom
"And the reaper receives wages and gathers fruit for eternal life." The fruit of the earthly harvest serves not eternal life, but temporal life. Spiritual fruit, by contrast, serves a life that does not grow old and is immortal.
Do you see that the words are sensible, but the meaning is spiritual? By the very form of the expressions, earthly things are separated from heavenly things.
When He spoke about water and described its properties, He said that whoever drinks of this water will never thirst. So here He says that this fruit is gathered for eternal life.
"So that both he who sows and he who reaps may rejoice together." Who is the sower, and who is the reaper? The prophets were the sowers, but they themselves did not reap. The Apostles reaped.
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Yet the prophets are not thereby deprived of joy or of the reward for their labors. They rejoice and exult together with us, even though they do not reap with us.
Harvest and sowing are not the same. Therefore I have kept you for that in which the labor is less and the consolation greater, not for sowing.
In sowing there is much hardship and toil. In harvest there is much gain, but the labor is not the same. It is easier.
By this He wishes to show that it was also the desire of the prophets that men be converted to Him. The law also prepared for this.
For this reason they sowed, to produce this fruit. He also shows that He sent the prophets, and that the New and Old Testaments have a great kinship with one another.
Through the present parable He makes all these things visible at once.