Jerome

For he that takes heed of the leaven of the Pharisees and the Sadducees, does not observe the precepts of the Law and of the letter, and neglects the traditions of men that he may do the commandments of God. This is the leaven of which the Apostle speaks, A little leaven leaveneth the whole lump. (1 Cor. 5:6.) By every means also we should avoid that leaven which Marcion, Valentinus, and all the heretics had. …

Catena Aurea by Aquinas ↗

Hilary of Poitiers

The apostles are ordered to watch out for the leaven of the Pharisees and Sadducees. They are warned not to be involved in the disputes of the Jews. The works of the law are now to be viewed in the light of faith. They are forewarned that they, into whose time and age the truth had appeared incarnate, should judge nothing except which lies within the position of hope in likeness of the truth that is revealed. …

Commentary on Matthew 16.3 ↗

Thomas Aquinas

Because he said: take heed and beware etc. Here the teaching is set forth. By leaven he means corrupt doctrine; hence he does not mean the doctrine of the law, but the traditions of the Pharisees, which are called leaven, because just as from a little leaven the whole is corrupted, so from a small error the whole of life is corrupted, just as from a road from which a man departs a little, he afterwards goes far astray: hence the Philosopher says in the first book of On the Heavens that a small error in the beginning becomes great at …

Commentary on Matthew ↗