Jesus teaches us to avoid the "leaven" of the Pharisees, meaning their hypocrisy. Leaven is an element that modifies or transforms a substance, particularly the dough that becomes bread. Leaven is that which makes the dough rise and it takes time. God tells the Israelites to use Unleavened bread for the Passover meal, due to the urgency in which the meal was to be celebrated. They couldn't wait for the leaven to do what it's supposed to do: to make the dough RISE.
The leaven of the Pharisees, hypocrisy, is much like the leaven used in bread baking: hidden. Hypocrisy is insidious and works, like yeast, to transform our inner attitudes and beliefs. We become DEformed by this kind of leaven. We can't RISE to living the Gospel, by loving all we encounter, by forgiving, by being selfless. Symbolically, leaven can be not only hypocrisy, but anything that corrupts our relationship with Christ: envy, jealousy, bitterness, aggression, racism, rigidity, miserly, over indulgence, the list goes on and on...
A few days ago I tried to make some bread and it would not RISE properly. There is nothing more deflating than seeing a loaf of bread flat! It turns out the yeast, the leaven, was stale and could no longer do what it is supposed to do. May the "leaven" of the Holy Spirit never become dull or lifeless within our hearts. Like delicious bread dough, we are called to RISE to the occasion of living the Good News of Jesus.
Peace. Joy. Fr. Frank