Here is a quote from one of the best commentators you can find, Cornelius a Lapide
Ver. 26. And He said, So is the kingdom of God, as if a man should cast seed into the earth. This is another parable, different from that of the Sower, which precedes it. Both, however, are taken from seed, but differently applied and explained. Moreover, by the seed, as SS. Chrysostom and Bede rightly explain, both here and in S. Matt. xiii., is signified evangelical doctrine. By the field, hearers; by the harvest, the end of the world, or each one’s death, is meant.
Ver. 27. And should sleep, that is to say, the sower, and rise, night and day, and the seed should spring and grow up whilst he knoweth not. Some refer the words rise night and day to the seed; meaning that the seed should germinate, it knoweth not how, that is, like a man sleeping. More obviously, S. Chrysostom, Theophylact, Maldonatus, and others refer the words to the sower, so that night pertains to the word sleep, day to the word rise. The meaning is, As the husbandman who has sowed is sleeping idly in the night, and is employed in various occupations during the day, and thinks not about the seed, that seed is germinating by its own innate force, and is growing up whilst the husbandman knoweth it not. So also it puts forth first the blade, then the ear, and then the full corn in the ear. So, too, in the same manner is the doctrine and preaching of the Gospel. They were sown by Christ and His Apostles, that is, they were preached from small beginnings. But by degrees they grew insensibly into the mature and mighty harvests of the faithful, whilst Christ was, as it were, sleeping in heaven, and permitting the Jews and unbelieving nations and tyrants to rise up against His Apostles, and persecute and kill them. It increases, I say, and propagates itself gradually, until it fills the world, when, the harvest being ripe, the corn, i.e., the elect, shall be gathered into the granary of heaven.
By this parable, then, is signified the power of the Gospel, which by degrees has pervaded the whole world, and is converting it to Christ. Tacitly, also, it is signified that preachers of the Gospel must not glory in their preaching, as though they by it were converting the world. For, as the Apostle saith, “Neither he that planteth is anything, nor he that watereth, but God that giveth the increase (1 Cor. iii. 7). Christ further intimates that preachers ought not to be downcast if they see small and tardy fruits of their preaching, because God will, by the few converted by them, gradually convert many more. So S. James, by means of seven, or, as some say, by nine, whom he converted in Spain, converted the whole country.
Ver. 28.* For the earth of itself bringeth forth fruit; first the blade, then the ear, afterwards the full corn in the ear. Arabic, Because the earth alone bringeth forth fruit; . . . afterwards the ear is filled, and when the fruit is perfect, then the sickle is applied, because it is harvest.* Thus, in like manner, by the preaching of the Gospel, the faith of Christ and His Church grew by various degrees of increase.
Moraliter: Expositors adapt these three expressions, blade, ear, full corn, to a threefold increment of virtues and merits. For the earth of our heart germinates, first, the blade, when it conceives good desires; secondly, the ear, when it proceeds to earnest working; thirdly, the grain, when it brings the works of virtue to full maturity and perfection. Theophylact says, “The blade is the beginning of good; the ear is when we resist temptations; the fruit is perfect work.”
Hear S. Gregory (Hom. 15, in Ezek.), “To produce the blade is to hold the first tender beginning of good. The blade arrives at perfection when virtue conceived in the mind leads to advancement in good works. The full corn fructifies in the ear when virtue makes such great progress that it has its perfect work.”
Christ here intimates that the Apostles, and those who work for the conversion of souls, ought with long-suffering to await the fruit of their labours, as husbandmen do. They ought to cherish those who are tender in the faith, and gradually lead them on to the height of virtue by teaching, admonishing, and exercising them. Let no one, therefore, says Bede, who is beheld to be of good purpose in the tenderness of his mind, be despised, because the fruit takes its rise from the blade, and becomes corn.
Symbolically: The Scholiast says the blade was in the law of nature, the ear in the law of Moses, the fruit in the Gospel.
Ver. 29. *And when the fruit is brought forth, immediately he pulleth in the sickle. *Greek, όταν δὲ παραδω̃ ό καρπός, that is, when indeed the fruit has brought itself forth; for fruit is here in the nominative case. The Syriac has, when it has become fat; Arabic, when it is perfect. This is a Hebraism, for in Hebrew verbs in the conjugation Hitpael have a passive, or reflex, signification, by which the agent receives the action in himself, so that the agent is the same as the recipient of the action. Wherefore some codices read, when the fruit has produced itself. Otherwise Maldonatus explains, “When the fruit, that is, the seed itself, which was the fruit of former seed, shall have brought forth, that is to say, other seed from itself.”