Albert Barnes Commentary Matthew 16:6-8

"And Jesus said unto them, Take heed and beware of the leaven of the Pharisees and Sadducees. And they reasoned among themselves, saying, We took no bread. And Jesus perceiving it said, O ye of little faith, why reason ye among yourselves, because ye have no bread?" — Matthew 16:6-8 (ASV)

Take heed. That is, be cautious, be on your guard.

The leaven of the Pharisees and of the Sadducees. Leaven is used in making bread. Its use is to pass through the flour, and cause it to ferment or to swell, and become light. It passes secretly, silently, but certainly, and no one can see its progress.

So it was with the doctrines of the Pharisees. They were insinuating, artful, plausible. They concealed the real tendency of their doctrines; they instilled them secretly into the mind, and they pervaded all mental capacities, like leaven.

They reasoned. The disciples did not understand Him as referring to their doctrine, because the word leaven was not often used among the Jews to denote doctrines, as no other instance of the use of this word occurs in the Scriptures. Besides, the Jews had many particular rules about the leaven that might be used in making bread. Many held that it was not lawful to eat bread made by the Gentiles, and the disciples perhaps supposed that He was cautioning them not to procure a supply from the Pharisees and Sadducees.

O you of little faith. Jesus, in reply, said that they should not be so anxious about the supply of their needs. They should not have supposed, after the miracles that He had performed in feeding so many, that He would caution them to be anxious about procuring bread for their necessities. It was improper, then, for them to reason about a thing like that, but they should have supposed He referred to something more important. The miracles had been full proof that He could supply all their needs without such anxiety.