“Beware of the leaven of the Pharisees” was a phrase Jesus used in Luke 12:1 as a warning against a particularly damaging and evil ailment characteristic of the religious leaders of His day: hypocrisy. Leaven makes a great illustrator of the properties that hypocrisy possesses, which causes so much damage in God’s people.
First, there is the fact that danger inheres in a foreign ingredient within a substance. Leaven in a lump of dough changes its nature. Further, it compromises its purity, since it is foreign substance in the bread-to-be. Also, over the course of time in the oven, it actually makes it something entirely different than what it was before. Finally, there is the implicit warning that this foreign substance has a nature about it: it propagates, it grows.
In every case where leaven is used in the negative sense (and most of the time, that is the way Jesus used it), it perfectly describes sin. Sin compromises our purity. When sin is allowed into our life, we become tainted and dirty, a compromised Christian. Further, it is the introduction of a substance foreign to what we should be as Christians. We are dead to sin (Romans 6:11) and only by our rejection of God or compromise of His will can sin again become alive in the child of God. Also, it has a nature about it that tends to grow. Satan may invite us for just a little but his intention is to inundate us to the degree that we cannot disentangle ourselves from the briars of sin (cf. Galatians 6:1). Satan is never satisfied with a little sin; he will not be happy until he has inundated the Christian with a total blackness of ungodliness.
The oddity in the above passage is that the proponents of this evil leaven were religious leaders. So, here is the lesson: study the Book for yourself! Trust in God, not men. And, understand that hypocrisy in high places can bring down a nation.