Matthew 16:12

“Then Jesus said unto them, Take heed, and beware of the leaven of the Pharisees and of the Sadducees;” and so forth, down to where we read, “Then understood they that He bade them not beware of the leaven of bread, but of the doctrine of the Pharisees and of the Sadducees.”” — editor's note: Matt. xvi. 5–12.

“Then understood they,” it is said, “that He called their doctrine leaven.”” — editor's note: Matt. xvi. 9, 12.

As the Lord had left the Pharisees on account of their unbelief, so now He teaches His disciples to be on their guard against their doctrine; whence it follows, “And when His disciples were come to the other side,…

They were bound to their Master with so great affection, that they were unwilling to part from Him for even a moment of time. And herein it should be observed how far they were from any longing for delicacies, when…

Herein the Apostles are admonished not to be partakers in the doctrine of the Jews; for the works of the Law were established to produce faith, and to prefigure the things that were to follow; and they on whose times…

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…

Why did He not say plainly, Take heed of the doctrine of the Pharisees? Because He would remind them of those things that had been done in the multiplication of the loaves, knowing them to be forgetful. To have given…

How had they no bread, seeing that as soon as they had filled seven baskets they entered into the boat, and came into the parts of Magedan? There they hear that they ought to take heed of the leaven of the Pharisees…

Because the disciples still grovelled about Jewish observances, the Lord sharply rebukes them for the benefit of all; whence it follows, “But Jesus knowing their thoughts said unto them, O ye of little faith, why…

As much as to say; Why do ye think that I spake of earthly bread, for which ye ought not to have a thought, having beheld Me of so little make such abundant overplus?

This He does that He may put away from them all care for food. But why did He not reprove them, when they said, “Whence should we have so much bread in the wilderness?” for that seemed a more fitting occasion. He did…

As much as to say, Do ye not understand the mystery, nor remember the miracle?

By this calling to mind what was past, and rousing their attention to what was to come.

Thus He takes this occasion to instruct them what is meant by the five loaves and the seven loaves, the five thousand and the four thousand, who were fed in the desert. For if the leaven of the Pharisees and…

But that you may learn what force Christ’s reproof had upon His disciples, and how it roused their sluggish spirit, hear what says the Evangelist; “Then they understood how that he bade them not beware of the leaven…

Matthew 16:12 and Mark 8:15 share the same wording (9 words).

Cited together in Jerome on Matthew 16:5–12 (Catena Aurea).