Yeast is the secret to making bread rise. When added to a lump of bread dough, the yeast ferments the starches in the dough and causes the dough to increase in size. Without yeast to leaven the dough, the loaf that comes out of the oven is flat. Remarkably, it takes little yeast to make dough rise. A small amount is sufficient to leaven dough thoroughly.
In today's passage, Jesus alludes to this phenomenon of yeast's making dough rise in order to teach a spiritual lesson. It is a natural image to use because our Lord has just fed four thousand people with seven loaves of bread, and the disciples are noting how they forgot to bring enough bread for a meal with them in the boat (Mark 8:1–10, 14–15). Jesus issues this warning: "Watch out; beware of the leaven of the Pharisees and the leaven of Herod."
Since it only takes a small amount of yeast or other leavening agent to transform an entire lump of dough, Jesus must mean that it takes only a little bit of what the Pharisees and Herod have to offer to ruin a person. We say "ruin" because Christ is giving a warning statement and because leaven almost always represents sin in the New Testament; only Jesus' parable of the leaven uses leaven in a positive sense (Matt. 13:33; Luke 13:20–21).
Just a smidgen of the leaven of the Pharisees and Herod suffices to transform what is good and useful into something bad and useless. But what does the leaven signify? That is harder to discern, for the Pharisees and Herod Antipas had little in common. The Pharisees were devoted to their study of the Mosaic law, and Antipas was anything but devout (see Mark 6:14–29). Christ may be referring to Herod and the Pharisees' contentment with Roman rule. Herod enjoyed political power because of Rome, and though many Jews wanted Rome overthrown, the Pharisees were basically fine with the Romans as long as Caesar allowed them to study the law in peace.
However, contentment with Roman governance does not fully capture what Jesus means by leaven. Unbelief in Christ is the leaven of the Pharisees and Herod, unbelief motivated in part by fear of Roman reprisal against the Jews if Jesus were acclaimed as Messiah. But their unbelief was ultimately due to their hard hearts (John 12:36b–43). Let us be on guard against hardness of heart; we might fall into spiritual ruin if we harden ourselves against God's Word even just a little bit.
When we harden our hearts even just a little bit and refuse to obey God on what we might consider a minor matter, we introduce leaven into our lives that can spread to other areas. It is imperative, therefore, that we regularly repent for our disobedience and seek to obey the Lord in all of life. We should be striving for obedience in all matters, not just the really "big" ones.