It is only one slight difference in an ingredient in breads. A tortilla or cracker is remarkable different from San Francisco sour dough bread. The difference is yeast.
Jesus is challenging me to understand that this is a spiritual issue, not a physical issue. I am to keep a sharp eye for the yeast of Religious Leaders. It doesn’t take much yeast for it to cause bread to rise. The result is remarkably different. So it was with the doctrines of the Religious Leaders. They are insinuating, artful, plausible. They conceal the real tendency of their doctrines; they instill them secretly into the mind, until they pervaded all my faculties like leaven.
I am responsible for knowing and evaluating whether the teaching of Religious Leaders has yeast in it or not. No one else can do it for me. I can see the difference. Is it a tortilla or sour dough bread? Jesus said “I am the bread of life; whoever comes to me shall not hunger, and whoever believes in me shall never thirst.”
On their way to the other side of the lake, the disciples discovered they had forgotten to bring along bread. In the meantime, Jesus said to them, “Keep a sharp eye out for the yeast of Religious Leaders (aka Pharisee-Sadducee).”
Thinking he was scolding them for forgetting bread, they discussed in whispers what to do. Jesus knew what they were doing and said, “Why all these worried whispers about forgetting the bread? Runt believers! Haven’t you caught on yet? Don’t you remember the five loaves of bread and the five thousand people, and how many baskets of fragments you picked up? Or the seven loaves that fed four thousand, and how many baskets of leftovers you collected? Haven’t you realized yet that bread isn’t the problem? The problem is yeast, the Religious Leader’s (aka Pharisee-Sadducee) yeast.” Then they got it: that he wasn’t concerned about eating, but teaching—the Religious Leaders (aka Pharisee-Sadducee) kind of teaching.
Source: Matthew 16:5-12