The leaven. Christ's thoughts were still fixed on the late disputants, whose powerful influence on popular opinion called for forcible warning. By "leaven" he does not here refer specially to the hypocrisy of the Pharisees and Sadducees, as in Luke 12:1, but to the evil influence which they exercised, which was diffused far and wide, and penetrated to all ranks and classes.
Their unsound opinions, their inability or disinclination to enter into the spiritual sense of Scripture, vitiated their whole system, and made them dangerous teachers directly they attempted to explain or amplify the letter of Holy Writ.
It was this same perverse blindness that led them to refuse to accept Jesus as Messiah in spite of all the proofs which had been brought before them. That leaven, in one aspect, was regarded as a sign of impurity and corruption, we learn from the strict rules which banished it from Divine service, and especially during the Passover season.
Says St. Paul, "A little leaven leaveneth the whole lump" (Galatians 5:9); and, "Purge out the old leaven, that ye may be a new lump, even as ye are unleavened" (1 Corinthians 5:7). Elsewhere Christ makes a distinction between what these teachers taught ex cathedra, and what they put forth on their own authority or what they practised themselves (Matthew 23:2, Matthew 23:3, where see note).