A little bit of yeast permeates the whole batch of dough [i.e., a few people can influence everyone else to do the wrong thing in this matter].

It is similar to yeast, which a woman worked into three batches of dough, until the dough was completely permeated [by it]."

Jesus said to them, "Pay attention and watch out for the leavening [effect] of the Pharisees and Sadducees."

How is it that you do not understand that I was not speaking to you about [physical] bread? But [rather I meant] watch out for the yeast [i.e., the influence] of the Pharisees and Sadducees."

Then Jesus ordered them, saying, "Pay attention and watch out for the leavening [effect] of the Pharisees and Herod." [Note: By "Herod" is probably meant the Herodians, who were a party favorable to King Herod and, along with the Pharisees and Sadducees, were out to get Jesus. See Matt. 16:6].

About this time, while a crowd of many thousands had assembled, [so large that] people were trampling on one another, Jesus first began telling His disciples, "Watch out for the leavening [effect] of the Pharisees, which is hypocrisy.

Then they understood that He was not telling them to watch out for the yeast in [physical] bread, but for the teaching of the Pharisees and Sadducees.

So, we should observe the Festival [i.e., live the Christian life], but not with the old yeast [i.e., old sinful ways], such as the leavening [effect] of evil and wickedness, but with the unleavened bread of sincerity and truth.

Then He told them another parable: "The kingdom of heaven is similar to yeast, which a woman worked into a batch of dough [i.e., the amount was over half a bushel], until [the dough] was completely leavened [by it]."

Your boasting is not good. Do you not know that a little yeast can leaven the whole batch of dough?