What does the Bible say about leaven and its significance?

In the Bible, particularly in Mark 8:13-21, Jesus warns His disciples to beware of the leaven of the Pharisees and Herod. Leaven is often used as a metaphor for sin or false doctrine, illustrating that even a small amount can affect the whole. The Pharisees' leaven represents self-righteousness and hypocrisy, while Herod's leaven symbolizes indulgence in worldly pleasures. As believers, we must recognize that the truths of the Gospel cannot be compromised by even a hint of false teaching.