Mark 8:14

Let us notice the solemn warning which our Lord gives to His disciples at the beginning of this passage. He says, "Take heed, beware of the leaven of the Pharisees and of the leaven of Herod." We are not left to conjecture the meaning of this warning. This is made clear by the parallel passage in Matthew's Gospel. We there read that Jesus did not mean the leaven of "bread," but the leaven of "doctrine." The self-righteousness and formalism of the Pharisees — the worldliness and skepticism of the courtiers of Herod, were the object of our Lord's caution. Against both He bids His disciples be on their guard. Such warnings are of deep importance. It would be well for the Church of Christ, if they had been more remembered. The assaults of persecution from without have never done half so much harm to the Church, as the rise of false doctrines within. False prophets and false teachers within the camp have done far more mischief in Christendom than all the bloody persecutions of the emperors of Rome. The sword of the foe has never done such damage to the cause of truth as the tongue and the pen. The doctrines which our Lord specifies, are precisely those which have always been found to inflict most injury on the cause of Christianity. Formalism on the one hand, and skepticism on the other, have been chronic diseases in the professing Church of Christ. In every age multitudes of Christians have been infected by them. In every age men need to watch against them, and be on their guard. 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.

Now the disciples had forgotten to take bread - See all this, to Mar 8:21, explained at large on Mat 16:4-12 (note). In the above chapter, an account is given of the Pharisees, Sadducees, and Herodians.