Jesus commanded them and said, “Watch out for the leaven of the Pharisees and the leaven of the Herodians.”
Here Christ lifts their thinking above the material matter of bread and directs it toward inner and spiritual dangers. The word “leaven” here symbolizes something small that, once it enters, spreads through the whole. The leaven of the Pharisees refers to hypocrisy, self-sufficiency, and outward religiosity without humility of heart. The leaven of the Herodians is connected with worldly thinking, when a person subordinates the truth to his own interests. Both are dangerous, because they work from within and can, unnoticed, gradually transform a person’s entire inner world. For this reason Christ says, “Be on guard,” because such things often do not appear in the form of open evil, but come as ordinary or acceptable things. This verse reminds us that the believer must be careful not only of obvious sin, but also of those inner influences that can quietly draw him away from the true way of Christ.