A pinch of yeast ferments the whole batch.

The disciples had forgotten to bring bread, and they had only one loaf with them in the boat. Jesus enjoined them, “Watch out, guard against the leaven of the Pharisees and the leaven of Herod.” - Mark 8:14-15

The setting for this discourse by the Lord with disciples comes after Jesus multiplies the loaves for the crowd and after he has the "give me a sign" run in with the Pharisees. Jesus left the Pharisees and crossed to the other shore by boat with his disciples. While they were crossing, the disciples noticed they had forgotten to bring some bread with them. At this point, Jesus pops off the warning to the disciples to beware of the leaven of the Pharisees and the leaven of Herod.

Now, I can only imagine the frame of mind that Jesus was in at this point, and I can only imagine what the disciples were thinking as well. Jesus is their leader, and he has just told off the Pharisees who were regarded very highly in the Jewish faith. We have all been in this position before, when say our supervisor and his boss exchange words in front of us, and we are left in an awkward and uncomfortable position. Then the supervisor pipes up with some statement that makes things only more awkward. This is what is happening here with Jesus and his disciples, so when Jesus warns them about the leaven of the Pharisees and Herod, I am sure that they were thinking, "Huh? What is this leaven he is talking about? Is it because we didn't bring any bread?"

Jesus then becomes even more flustered because it would seem that the disciples have completely missed the significance of all the events that they have just witnessed. As Jesus says to them, "Why are you talking about having no bread? Do you still not understand, still not realize? Are your minds closed? Have you eyes and do not see, ears and do not hear?" The disciples are focused on the materialistic aspects of the miracles, that of bread and fish rather than the compassion of Christ and his feeding of the hungry crowd with multiple baskets of bread left over, an image of the overflowing nature of God's love and mercy.

So what of the leaven of the Pharisees and the leaven of Herod? Leaven or yeast has a corrupting action on bread due to fermentation which makes the bread rise. In the process, the bread becomes more palatable and easier to digest. When Jesus speaks of the leaven of the Pharisees and Herod, he is speaking of the corruption of God's truth which would make it easier for the listener to accept. So, the leaven of Herod would be that of the decadent hedonistic world in which prudence and sound judgment are thrown to the wind, and the desires of the flesh are put above all else. But the leaven of the Pharisees is perhaps more dangerous because it is the outward show of holiness which serves the self before God or others, i.e. hypocrisy. Both of these scenarios are at odds with the gospel which is love that is selfless and sacrificial in nature.

This is the point that Jesus is trying to drive home with the disciples. It is the same lesson that he teaches to all generations. Thank heaven he is patient with our ignorance!

Through the intercession of Saint Cyril and Saint Methodius, may Christ our God have mercy on us and save us.