Charles Ellicott Commentary Matthew 16:6

"And Jesus said unto them, Take heed and beware of the leaven of the Pharisees and Sadducees." — Matthew 16:6 (ASV)

Beware of the leaven of the Pharisees — The form of this warning was clearly determined by the event that had just occurred. The Master saw the disciples’ perplexed looks and heard their self-reproaching or mutually accusing whispers, and He used their confusion as the basis for a proverb that was like a condensed parable.

As St. Mark records the words, they are: Beware of the leaven of the Pharisees, and of the leaven of Herod. If we must choose, we can believe this was the form in which they were actually spoken. St. Matthew, or the source he followed, substituted the better-known Sadducees for the less-familiar Herodians.

The tetrarch's language, as has been shown (see note on Matthew 14:2), implies that Sadduceeism had been the prevailing belief of his life. Consequently, the current of Jewish political—and even religious—sympathies naturally led the Sadducean priests to associate with the scribes attached to the tetrarch's party. These priests, like Caiaphas, were seeking the favor of the Roman rulers .