Matthew 16:12
Then understood they how that he bade them not beware of the leaven of bread, but of the doctrine of the Pharisees and of the Sadducees. Then understood they how that he bade them not beware of the leaven of bread, but of the doctrine of the Pharisees and of the Sadducees.
The Pharisees and the Sadducees stood in bitter enmity toward each other, yet in the setting of this warning they united against Christ. The disciples came to understand that when He bade them beware of leaven He meant not leaven of bread but the doctrine or teaching of the Pharisees and of the Sadducees.
That leaven is the maxims and customs and opinions of the Pharisees, which like leaven at work would mislead the new converts who would be added to the church. To counter that influence Christ gave His disciples parables containing great truths to have an opposite leavening effect, truths to be remembered with distinctness afterward and then vividly brought to the minds of those new converts. Though addressed to His disciples, the warning also spoke to the Pharisees themselves, many of whom were deeply convicted as they heard living testimony given under the dictation of the Holy Spirit, with the desire that if not before His crucifixion then after His resurrection and ascension they would be converted from Phariseeism and receive the truth.
Unbelief toward the teachings of the Spirit of God is that leaven. It insinuates itself until it spreads generally through the church, sickening spirits and crippling energies. Then the majority no longer receives faithful testimony, and there is no freedom left to bear it.