Jesus said to the crowds, “This is how it is with the Kingdom of God; it is as if a man were to scatter seed on the land and would sleep and rise night and day and the seed would sprout and grow, he knows not how. Of its own accord the land yields fruit, first the blade, then the ear, then the full grain in the ear. And when the grain is ripe, he wields the sickle at once, for the harvest has come.”
He said, “To what shall we compare the Kingdom of God, or what parable can we use for it? It is like a mustard seed that, when it is sown in the ground, is the smallest of all the seeds on the earth. But once it is sown, it springs up and becomes the largest of plants and puts forth large branches, so that the birds of the sky can dwell in its shade.” With many such parables he spoke the word to them as they were able to understand it. Without parables he did not speak to them, but to his own disciples he explained everything in private.
From the parable of the sower, we heard about the three seeds which perished in various ways, and the one which was saved; in which Jesus also shows three differences, according to the proportion of faith and practice. Here however, He puts forth a parable concerning only those who are saved. Therefore it is said, "And he said, So is the kingdom of God, as if a man should cast seed into the ground, etc."
Man casts seed to the ground, when he places a good intention in his heart; and he sleeps, when he already rests in the hope which attends on a good work. But he rises night and day because he advances amidst prosperity and adversity, though he does not know it for he is as yet unable to measure his increase, and yet virtue, once conceived, goes on increasing. Therefore when we conceive good desires, we put seed into the ground; when we begin to work rightly, we are the blade. When we increase to the perfection of good works, we arrive at the ear; when we are firmly fixed in the perfection of the same working, we already put forth the full corn in the ear.
Catena Aurea (St. Thomas Aquinas)
Homily 2 on Ezekiel (St. Gregory the Great)