Matthew 16:6

The leaven of the Pharisees and of the Sadducees] St. Mark says, 'of the Pharisees and of Herod.' Herod may have been a Sadducee in spite of his superstitious belief in John's resurrection, but, even if he was not, he exactly represented the secular, irreligious, worldly spirit of Sadduceeism. The leaven of the Pharisees is hypocrisy, ostentation, pride, formalism, scrupulosity, and the tendency to place the letter before the spirit. The leaven of the Sadducees is worldliness, and the temper of irreligious scepticism.

The disciples took Jesus' words literally as a command to lay in a fresh stock of bread, taking special precautions to avoid all bread made with leaven from the house of a Pharisee or a Sadducee. The misunderstanding is not so absurd, if it be remembered that Gentile food and Gentile leaven were regarded by the stricter Jews as unclean. Since Jesus had pronounced the Pharisees worse than the heathen, it was quite natural (from the strictly Jewish point of view) that He should proceed to pronounce their houses, food, and, therefore, their leaven unclean. Jewish writings contain subtle discussions as to when, why, and under what circumstances heathen, Samaritan, and Christian leaven is to be regarded as unclean.

9, 10. See on Matthew 15:32.