Charles Spurgeon Commentary Matthew 16:12

"Then understood they that he bade them not beware of the leaven of bread, but of the teaching of the Pharisees and Sadducees." — Matthew 16:12 (ASV)

The doctrine of these sectarians had a secret, insidious, and corrupting influence, and the disciples had to watch carefully in case even a little of its spirit and teaching got into their minds and then spread throughout their whole nature. Both these kinds of leaven can be at work simultaneously in the same community. In fact, they are essentially one leaven. The two sets of opponents assailed the Lord Jesus at the same time, because they had a common ground of opposition against Him.

To this day, these two forms of evil are continually working, either secretly or openly, and we need to be wary of them at all times. It is important to understand this and to both purge out the old leaven of Pharisaism and keep out the new leaven of Sadduceeism. Self-righteousness and carnal reasoning must both be cast out. Faith will find them both to be her deadly foes.

Many are entertaining this evil leaven, and before they realize it, the unhallowed thing will defile them. To be evangelical and yet to be superstitious or rationalistic at the same time is nearly impossible. Some of our contemporaries are trying to bake with this leaven, but their bread will be sour. BEWARE!