Charles Ellicott Commentary Matthew 16:12

"Then understood they that he bade them not beware of the leaven of bread, but of the teaching of the Pharisees and Sadducees." — Matthew 16:12 (ASV)

The doctrine of the Pharisees and of the Sadducees — This is better understood as teaching, referring not so much to the formulated dogmas of the sects as to their general spirit and tendency. As Luke 12:1 expressly states, the leaven of the Pharisees was hypocrisy. This refers to the unreality of a life that is respectable, rigid, outwardly religious, and even earnest in its zeal, yet lacks the humility and love that are the essence of true holiness.

The leaven of the Sadducees and of Herod, we may believe (as it is not specially defined), was the more open form of worldliness and self-indulgence. This worldliness was connected to their denial of the resurrection and, therefore, of eternal life.