Leaven or “yeast” in the Bible speak of influence. When you add a little yeast to a lump of dough, within a few hours the entire batch has bubbled and risen. In the same way Jesus said in the Bible passage below to beware of the leaven of the Pharisees (legalism) and the leaven of Herod (libertinism).
The Pharisees felt that keeping rules and regulations would get them to heaven, but actually it kept them from having a real relationship with God. The Herodians on the other hand had totally adopted the sinful lifestyle of the Roman Empire. And, people to fall prey to the influence of sin are in danger of missing God’s plan for their life.
(Mark 8:14-21) 14 The disciples had forgotten to bring bread, except for one loaf they had with them in the boat. 15 “Be careful,” Jesus warned them. “Watch out for the yeast of the Pharisees and that of Herod.” 16 They discussed this with one another and said, “It is because we have no bread.”
17 Aware of their discussion, Jesus asked them: “Why are you talking about having no bread? Do you still not see or understand? Are your hearts hardened? 18 Do you have eyes but fail to see, and ears but fail to hear? And don’t you remember?
19 When I broke the five loaves for the five thousand, how many basketfuls of pieces did you pick up?” “Twelve,” they replied. 20 “And when I broke the seven loaves for the four thousand, how many basketfuls of pieces did you pick up?” They answered, “Seven.” 21 He said to them, “Do you still not understand?”