Now they had forgotten to bring bread, and they had only one loaf with them in the boat. And he cautioned them, saying, “Watch out; beware of the leaven of the Pharisees and the leaven of Herod.” And they began discussing with one another the fact that they had no bread. And Jesus, aware of this, said to them, “Why are you discussing the fact that you have no bread? Do you not yet perceive or understand? Are your hearts hardened? Having eyes do you not see, and having ears do you not hear? And do you not remember? When I broke the five loaves for the five thousand, how many baskets full of broken pieces did you take up?” They said to him, “Twelve.” “And the seven for the four thousand, how many baskets full of broken pieces did you take up?” And they said to him, “Seven.” And he said to them, “Do you not yet understand?”
J.C. Ryle comments at “Grace Gems!”
They thought that the “leaven” of which He spoke must be the leaven of bread. It never struck them that He was speaking of doctrine. They drew from Him the sharp reproof–“Perceive you not yet, neither understand? are your hearts hardened? How is it that you do not understand?” Believers, converted, renewed, as the disciples were, they were still dull of apprehension in spiritual things. Their eyes were still dim, and their perception slow in the matters of the kingdom of God.
We shall find it useful to ourselves to remember what is here recorded of the disciples. It may help to correct the high thoughts which we are apt to entertain of our own wisdom, and to keep us humble and lowly-minded. We must not fancy that we know everything as soon as we are converted. Our knowledge, like all our graces, is always imperfect, and never so far from perfection as at our first beginning in the service of Christ. There is more ignorance in our hearts than we are at all aware of. “If any man think that he knows anything, he knows nothing yet as he ought to know.” (1 Cor. 8:2.)