Matthew 16:5 When the disciples reached the other side, they had forgotten to bring any bread. Jesus said to them, “Watch and beware of the leaven of the Pharisees and Sadducees.” And they began discussing it among themselves, saying, “We brought no bread.” But Jesus, aware of this, said, “O you of little faith, why are you discussing among yourselves the fact that you have no bread? Do you not yet perceive? Do you not remember the five loaves for the five thousand, and how many baskets you gathered? Or the seven loaves for the four thousand, and how many baskets you gathered? How is it that you fail to understand that I did not speak about bread? Beware of the leaven of the Pharisees and Sadducees.” Then they understood that he did not tell them to beware of the leaven of bread, but of the teaching of the Pharisees and Sadducees.
Luke 12:1 In the meantime, when so many thousands of the people had gathered together that they were trampling one another, he began to say to his disciples first, “Beware of the leaven of the Pharisees, which is hypocrisy.
1 Corinthians 5:6-8 Your boasting is not good. Do you not know that a little leaven leavens the whole lump? Cleanse out the old leaven that you may be a new lump, as you really are unleavened. For Christ, our Passover lamb, has been sacrificed. Let us therefore celebrate the festival, not with the old leaven, the leaven of malice and evil, but with the unleavened bread of sincerity and truth.
Galatians 5:9 A little leaven leavens the whole lump
2 Timothy 2:16-17 But avoid irreverent babble, for it will lead people into more and more ungodliness, and their talk will spread like gangrene.
Matthew 16:12 Then they understood that he did not tell them to beware of the leaven of bread, but of the teaching of the Pharisees and Sadducees.
“It was the Jewish metaphorical expression for an evil influence. To the Jewish mind leaven was always symbolic of evil…leaven stood for an evil influence liable to spread through life and to corrupt it.” (Barclay)
“False doctrine; which is fitly called leaven, because it soureth, swelleth, spreadeth, corrupteth the whole lump, and all this secretly.” (Trapp)
Then Jesus said, Beware of the leaven of the Pharisees — That is, of their false doctrine; elegantly called leaven, for it spreads in the soul, or the church, as leaven does in bread. (Benson)
It passes secretly, silently, but certainly through the mass of dough. None can see its progress.” So it was with the doctrines of the Pharisees. They were insinuating, artful, plausible. They concealed the real tendency of their doctrines; they instilled them secretly into the mind, until they pervaded all the faculties like leaven. It is secret, silent, steady; pervading all the faculties of the soul and all the kingdoms of the world, as leaven, or yeast, though hidden in the flour, and though deposited only in one place, works silently until all the mass is brought under its influence. (Barnes)
When God’s Word is neglected or when we become complacent with its influence in our daily lives, it is then we will begin to tolerate (by intentional choice) worldly or fleshly thoughts or actions that had no room in our heart, mind, and soul previously. By His Word and through the Holy Spirit we are given discernment into what is right, pure, good, and holy. By its neglect, we allow leaven in and day by day it silently and surely influences what we say, think, and do. How sad it is that through neglect and complacency we intentionally choose to become blind and deaf to things of God and the quiet whispers of the Holy Spirit’s leading in our lives. None are immune to the effects of “leaven”. The key is to be ever seeking and desiring the Holy Spirit’s leading through God’s Word so that in all things we think, say, and do, they honor and glorify Jesus Christ.