Mat 16:11 How is it that you do not understand that I spoke [it] not to you concerning bread, that you should beware of the leaven of the Pharisees and of the Sadducees?

At this point in the Gospels, Christ expects us to understand the nature of the physical, mental, emotional, and spiritual. His use of symbols in his language is very precise. Unlike human writers, he is perfectly consistent. Bread is a symbol (the main one) for the physical. Leaven is a symbol for the spiritual. The physical arises from the spirtual. Christ use leaven to describe the kingdom of heaven that changes everything by changing the spirit within it. However, the same is true of the spirit of the priest and powerful, the worldly spirit, which also mixes into everything.

This might be a good time to discuss the distinction that Christ draws between spiritual ideas and mental thoughts. Spiritual ideas or concepts exist independent of any real person or thing. The number "two,"for example, exists as an concept independent of any two things. It is perfectly real even without tangible form. Such concepts are real without form even if no one knows them. For example, the number "pi" existed even before people knew about it an used it. Spiritual ideas inspire and shape thoughts, but they are not the thoughts themselves. Once people know about "pi" the can use the idea and it can shape their actions.This is what Christ means by "spirit," the real concepts underlying reality.

Only God's concepts underly physical reality. When we get into the realm of relationships, the emotional, God's concepts contend with human concepts. Christ's role is to bring the kingdom of heaven, that is, God's concepts, back to human relationships and human society. This is his "leaven." These concepts must contend against those of organized religion and the state. He is warning the apostles that it is very easy to mix spiritual and worldly ideas.

People mix up Christ's ideas with worldly ideas to all the time. Christ, on the other hand, taught that religion and politics don't mix. Period. Religion is about your personal relationship with God not about reforming society. Christ says this a hundred different ways. If individuals embrace God through Christ's ideas, we will automatically reform society. This can only be done without the coercion of government. However, it this does not mean that people simply ignore what is wrong. It means that they confront it and challenge it on the basis of their faith.

"Understand" is from noeô, which means "to perceive with the eyes," "to perceive with the mind," and "to observe."

"Bread" is from artos, the same word translated as "loaves" in the previous verses.

"Beware" is from prosechô, which means "to hold to," "to offer," "to turn toward a thing," and "to pay attention." Only in the NT is it ever translated as "beware."