The Parable of the Weeds among the Wheat
In the parable the Lord spoke to such as do not receive the word of God; here of those who receive a corrupting seed. This is the work of the Devil, ever to mix error with truth.
Jesus says, "While men slept", for while the heads of the Church were asleep, and after the Apostles had received the sleep of death, then came the Devil and sowed upon the rest those whom the Lord in His interpretation calls evil children. But as Jesus interprets the field to mean the world, we may well understand them to be heretics, who in this world are mingled with the good. Indeed it is not necessary that every heretic or schismatic should be corporally severed from the Church; for the Church bears many who do not so publicly defend their false opinions as to attract the attention of the multitude, which when they do, then are they expelled. When then the Devil had sown upon the true Church many evil errors and false opinions.
And what are we laymen to do? When Jesus says, "Sowed good seed", He intends that good will which is in the elect; when He adds, "An enemy came", He means that watch should be kept against him; when as the weeds grow up, He suffers it patiently, saying, "An enemy has done this", He recommends to us patience; when He says, "Lest in gathering the weeds, etc." He sets us an example of discretion; when then He says, "Suffer both to grow together till the harvest" He teaches us long-suffering; and, lastly, He executes justice, when He says, "Bind them into bundles to burn."
Reference:
Catena Aurea (St. Thomas Aquinas)
Quaestionum Septemdecim In Evangelium Secundum Matthaeum (St. Augustine)
Homily 46 on Matthew (St. John Chrysostom)
Commentariorum In Matthaeum (St. Rabanus Maurus)