False Teaching Creeps In

Matthew 16:8-12 
8 But Jesus, being aware of it, said to them, “O you of little faith, why do you reason among yourselves because you have brought no bread? 9 Do you not yet understand, or remember the five loaves of the five thousand and how many baskets you took up? 10 Nor the seven loaves of the four thousand and how many large baskets you took up? 11 How is it you do not understand that I did not speak to you concerning bread?—but to beware of the leaven of the Pharisees and Sadducees.” 12 Then they understood that He did not tell them to beware of the leaven of bread, but of the doctrine of the Pharisees and Sadducees.

Jesus warned of bad teaching, but the disciples did not get the allusion of yeast spoiling the bread.  If we allow bad yeast in the dough, it quickly multiplies and taints the entire loaf.  So if we allow rules of man or religious teachings that contradict what the Bible shows God says, we allow ourselves and the church to become unpalatable.  Our God and His word within us must remain pure, and we must contend earnestly for the faith.