Jesus called the doctrine of the Pharisees and Sadducees leaven. He did this not to confuse his disciples but to illustrate the effects of bad doctrine.

First, he upbraided the disciples for their lack of understanding (v. 9, 11). If they had needed bread, Jesus could have handled that; they would not have had to eat the leavened bread of the Pharisees. He fed the 5000 and the 4000 with barely enough food to feed one person, and yet they had baskets full of leftovers.

Then he taught them that the doctrine of the Pharisees was like leaven because it demonstrated similar characteristics to leaven:

• Leaven was forbidden with the Jewish sacrifices (Lev 2:11) identifying it as something God didn’t want mixed in with things offered to him. Thus, God wouldn’t want the bad doctrine of the Pharisees mixed in with the good doctrine of the disciples.
• Leaven is hidden in the dough to make it rise (Matt 13:33), thus the bad doctrine of the Pharisees would be hidden in good doctrine so subtly that they might know it was there.
• Leaven leavens the whole lump (Matt 13:33), thus the bad doctrine of the Pharisees would eventually corrupt all of the good doctrine of the disciples.
• It only takes a little leaven to affect the whole lump (Gal 5:9), likewise it only takes a little bad doctrine to fully corrupt good doctrine

The doctrine of the Pharisees affects two primary areas: wrongly dividing the word of truth (2 Tim 2:15) and maintaining a careless attitude toward sin (1 Cor 5:1-6).

To prevent bad doctrine from corrupting your good doctrine you need to do the following:

• Rightly divide the word of truth (2 Tim 2:15).
• Get under some good preaching and teaching of the word of God (1 Tim 4:5-6).
• Take heed to yourself and your doctrine (1 Tim 4:16).
• Quit following every wind of doctrine (Eph 4:11-15).
• Guard yourself against smooth talking preachers with new doctrines (contrary doctrines) (Rom 16:16-17).

If you have already been exposed to bad doctrine, add salt. Salt stops the work of leaven (Mk 9:50; Col 4:6).