Then Jesus said unto them, Take heed and beware of the leaven of the Pharisees and of the Sadducees.
William Gurnall explains,
“[Just] for the secret mixture of it with the wholesome bread. You do not make your bread all of leaven, for none would then eat it, but crumble a little into a whole batch, which sours all. Thus Christ doth tell the disciples, that the Pharisees among many truths mix their errors; and therefore it behoves them to beware, lest with the truth the errors go down also. Again, leaven is very much like the dough, of the same grain with it, [and] only differs in age and sourness. Thus Christ intimates the resemblance of their errors to the truth, as it were, out of the Scriptures, but soured with their own false glosses. This indeed makes it easy for Christ’s sheep to be infected with the scab of error, because that weed which breeds the rot is so like the grass that nourisheth them.”