In the meantime, when an innumerable multitude of people had gathered together, so that they trampled one another, He began to say to His disciples first of all, “Beware of the leaven of the Pharisees, which is hypocrisy.”
During the Feast of Unleavened Bread, leavening represents sin that spreads subtly and puffs us up. Jesus highlighted one aspect of sin in the example of the religious leaders of His day who did much of what they did to show off. This attitude of hypocrisy not only involves lying and pride, but damages relationships with others and with God. Jesus Christ gave an extended warning against hypocrisy in powerful language in Matthew 23.
Read more about leaven in “What Is Leaven and What Does It Picture?”