Mark 8:1-26 (AD 29)
Alcoholic fermentation involves the digesting of sugar by yeast. It produces ethyl alcohol and carbon dioxide, which is how bread gets some of its flavour and all of its not-flatness. Trapped by the flexible dough, all the gas doesn’t disperse or break it’s surroundings, it just pushes it out. That’s the ‘rising’ of the bread, when we leave it in the airing cupboard for hours. If yeast is exposed to light, the light breaks apart the cell membrane and destroys the yeast.

The false teaching of the Pharisees, kneaded into the community for hundreds of years, like yeast, worked best in the dark. Left undisturbed, even in tiny little doses, it worked its way into not just the thoughts but the subconscious of everyone, leaving them changed.
The unrisen (unleavened, flat) bread that the Hebrews baked on Passover night that first time, before the Exodus, became a symbol of purity and purifying holiness, and of readiness to move in obedience to God’s commands. That night, there was no time to let your bread rise in a dark place.
In verses fourteen to twenty-one of our reading, Jesus warned his disciples against “the yeast of the Pharisees and that of Herod” (v.15).
He wasn’t warning them against their teachings so much – those were merely a symptom. He was warning them against their attitudes.
Herod’s response to a misunderstanding about who Jesus was, led him to feel threatened. Feeling threatened led him to murdering possibly dozens of babies and toddlers in case one of them would try and kick him off his throne when they grew up.
Herod’s response to niece’s sexy dancing was to promise her anything she wanted, and when she wanted someone murdered, his priority of saving face in front of his guests – his pride – led him to do as she asked.
The Pharisees arrogance and desire for control meant they persecuted Jesus as soon as he got popular, and because he criticised them. Their pride refused to even consider the possibility that Jesus was who he claimed to be.
Pride, a desire for power and a desire for control are a pejorative spiritual yeast. It doesn’t take much of either, but when they are left, undisturbed and unexamined, they expand and take over, changing the very nature of the thing that surrounds them.
As it turned out, Jesus’ warning was as well-founded as it was ignored. The disciples pride and desire for power led to Peter rebuking Jesus (8:32), all of them arguing about who was the greatest of them (9:32-33), stopping someone from driving out demons because he wasn’t one of them (9:38), and two of them asking for special honour in heaven (10:35-37).
Pride and a desire to grab or retain control over others aren’t flashy sins on their own. They’re not crimes. But left on their own, unchecked by accountability, unrepented of, unnoticed because we don’t ask God to show us our sins, it spreads through our entire personality. Over time it comes to affect every decision we make, every conversation.
Ask God to help you deal with the single-celled fungi of the Pharisees and Herod, to shine a purifying light on the yeast of your pride and desire for control, and stop it spreading. Understand, as the disciples initially struggled to, that Jesus really is the miracle-working God, Lord of all creation, saviour of humankind.