Beware of the leaven of the Pharisees and the leaven of Herod. St. Matthew (Matthew 15:6) says, "Beware of the leaven of the Pharisees and Sadducees;" thus St. Mark identifies the leaven of the Sadducees with that of Herod.
"Leaven" here means "doctrine." They were not to beware of this, so far as the Pharisees rightly taught and explained the Law of Moses; but only so far as they corrupted that Law by their own vain traditions, contrary to the Law of God, St.
Luke (Luke 12:11) calls this leaven "hypocrisy;" because the Pharisees only regarded outward ceremonies, and neglected the inward sanctification of the Spirit. St. Jerome says, "This is the leaven of which the Apostle speaks where he says, 'A little leaven leaveneth the whole lump.'
Marcion and Valentinus and all heretics have had this kind of leaven, which is on every account to be avoided. Leaven has this property, that, however small it may be in quantity, it spreads its influence rapidly through the mass.
And so if only a little spark of heretical doctrine be admitted into the soul, speedily a great flame arises, and envelopes the whole man."