Matthew 13:26

The Lord points out that our foe the devil sows the weeds of his wickedness and malice to choke the seed of God in us. Thus he says, "But while men were asleep, his enemy came and sowed weeds among the wheat and went away." The Lord indicates that the devil sows weeds among sleeping people—that is, among those who through negligence are overcome by their infidelity as in a kind of lethargy and fall asleep amid the divine injunctions. The apostle says concerning them: "For they who sleep, sleep at night, and they who are drunk, are drunk at night. Therefore let us not sleep as do the rest, but let us be wakeful and sober."Those foolish virgins about whom we read in the Gospel, weighed down by their lethargy and infidelity, not having taken oil for their vessels, were unable to go forth and meet the bridegroom. Hence it is always uppermost in the mind of this devil—the enemy of the human race—to sow weeds among the wheat.
But he who awaits the Lord faithfully, once the sleep of infidelity has been banished from him, will not be bothered by this nighttime sower.… According to the Lord's interpretation, the good seed represents the children of the kingdom and the weeds represent the wicked children.

And not by this only, but by what follows likewise, He depicts exactly all their acting. For, "When the blade was sprung up," saith He, "and brought forth fruit, then appeared the tares also;" which kind of thing these men also do. For at the beginning they disguise themselves; but when they have gained much confidence, and some one imparts to them the teaching of the word, then they pour out their poison.

And it should be noted that, when He says, Sowed good seed, He intends that good will which is in the elect; when He adds, An enemy came, He intimates that watch should be kept against him; when as the tares grow up, He suffers it patiently, saying, An enemy hath done this, He recommends to us patience; when He says, Lest haply in gathering the tares, &c. He sets us an example of discretion; when He says, Suffer both to grow together till the harvest, He teaches us long-suffering; and, lastly, He inculcates justice, when He says, Bind them into bundles to burn.

In the previous parable He spoke of the fourth part of the seed which fell on the good soil, while in this parable He shows that the enemy does not allow even that part which fell on good soil to remain incorrupted, because we sleep and grow indolent. The field, then, is the world, or, each one's soul. The sower is Christ. The good seed is good people, or, good thoughts. The tares are heresies, or, evil thoughts. The one who sows them is the devil. The men who were sleeping are those who by their indolence give entry to heretics and evil thoughts. The servants are the angels, who are indignant that there are heresies or any wickedness in the soul, and wish to seize and cut off from this life the heretics and those who think evil thoughts. But God does not allow the heretics to be destroyed by wars, lest the righteous suffer and be destroyed along with them. Likewise, neither does God wish to cut down a man on account of his evil thoughts, lest the wheat be destroyed along with them. If, for example, Matthew had been cut down while he was a tare, the wheat of the word which was later to spring up from him would have been cut down with him. Similarly with Paul and the thief. While they were tares they were not cut down, but were permitted to live so that later their virtue might grow. Therefore He says to the angels, At the end of the world you will gather the tares, namely, the heretics. But how? Into bundles, that is, binding them hand and foot. For at that time a man will no longer be able to do anything, but all his power to act will be bound. The wheat, namely, the saints, will be gathered by the angelic reapers into heavenly granaries. So it is with the evil thoughts which Paul had when he persecuted: they were burnt in the fire which Christ came to light upon the earth, while the wheat, that is the good thoughts, was gathered into the granaries of the Church.

Consequently, the progress of good and evil is treated. And when the blade was sprung up. And that you may understand, three things are considered. First, the manifestation of the good from the evil is set down; secondly, the zeal of the good against the evil; thirdly, tolerance. He says therefore, when the blade had sprung up and had brought forth fruit, then appeared the cockle also etc. For at the beginning, when it is sown, it does not appear, but when the blade grows. And this can be referred to both, both to the wheat and to the cockle. Augustine expounds it of the wheat, because when a man is small, he cannot discern; but when he grows and bears fruit and becomes spiritual, then he knows; 1 Corinthians 2:15: the spiritual man judges all things. Chrysostom expounds it of the cockle, because at first it does not appear, because heretics at first hide their teaching, because first they say and preach certain good things to the laity, and afterwards introduce certain evil things about the clergy, which are willingly heard; and so they turn the people away from love of the clergy, and so consequently from the Church. But afterwards, when they gain acceptance for their doctrine, they manifest their malice. For at first they say nothing but mild things, but afterwards they manifest themselves and their doctrine, which is understood by the wine; concerning which wine Proverbs 23:31 says: it goes in pleasantly, but in the end it will bite like a serpent.