Mark 8:15

the leaven of the Pharisees Leaven in Scripture, with the single exception of the Parable (Matthew 13:33; Luke 13:20-21), is always a symbol of evil (comp. 1 Corinthians 5:6-8; Galatians 5:9), especially insidious evil, as it is for the most part also in the Rabbinical writers. See Lightfoot on Matthew 16:6. The strict command to the children of Israel that they should carefully put away every particle of leaven out of their houses during the Passover-week, rests on this view of it as evil.

the leaven of Herod "and," as it is in St Matthew's Gospel, "of the Sadducees." The leaven of the Pharisees was hypocrisy(Luke 12:1), of the Sadducees, unbelief, of Herod, worldliness;all which working in secrecy and silence, and spreading with terrible certainty, cause that in the end "the whole man is leavened," and his whole nature transformed.