Now they had forgotten to bring bread, and they had only one loaf with them in the boat. 15 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?”
—Mark 8:14-21
J.C. Ryle comments at “Grace Gems!”
The expression used by our Lord in speaking of false doctrine is singularly forcible and appropriate. He calls it “leaven.” No word more suitable could have been employed. It exactly describes the small beginnings of false doctrine–the subtle quiet way in which it insensibly pervades a man’s religion–the deadly power with which it changes the whole character of his Christianity. Here, in fact, lies the great danger of false doctrine. If it approached us under its true colors, it would do little harm. The great secret of its success is its subtlety and likeness to truth. Every error in religion has been said to be a truth abused.
Let us often “examine ourselves whether we be in the faith,” and beware of “leaven.” Let us no more trifle with a little false doctrine, than we would trifle with a little immorality or a little lie. Once admit it into our hearts, and we never know how far it may lead us astray. The beginning of departure from the pure truth is like the letting out of waters–first a drop, and at last a torrent. A little leaven leavens the whole lump. (Gal. 5:9.)
Knowing God through His Word ... Day by Day 