Leaven, in Scripture, is always a picture of sin. But it is more than just that. In Matthew 13:33, Jesus said:
The kingdom of heaven is like leaven, which a woman took, and hid in three pecks of meal, until it was all leavened.
Likewise, in Luke 13:20-21, He said:
To what shall I compare the kingdom of God? It is like leaven, which a woman took and hid in three pecks of meal, until it was all leavened.
If we are familiar with the book of Revelation, we know that a woman represents a church. So what exactly is leaven?
And Jesus said to them, "Watch out and beware of the leaven of the Pharisees and Sadducees." And they began to discuss among themselves, saying, "It is because we took no bread." But Jesus, aware of this, said, "You men of little faith, why do you discuss among yourselves that you have no bread? Do you not yet understand or remember the five loaves of the five thousand, and how many baskets you took up? Or the seven loaves of the four thousand, and how many large baskets you took up? How is it that you do not understand that I did not speak to you concerning bread? But beware of the leaven of the Pharisees and Sadducees." Then they understood that He did not say to beware of the leaven of bread, but of the teaching of the Pharisees and Sadducees. Matthew 16:6-12
And He was giving orders to them, saying, "Watch out! Beware of the leaven of the Pharisees and the leaven of Herod." Mark 8:15
Under these circumstances, after so many thousands of the multitude had gathered together that they were stepping on one another, He began saying to His disciples first of all, "Beware of the leaven of the Pharisees, which is hypocrisy." Luke 12:1
Leaven, then, is hypocritical doctrine (teaching) and the seeking of signs. The Pharisees, the Sadducees, and Herod all wanted to see signs performed by Jesus. Jesus did not interact with Herod (Luke 23:9), and He walked away from the Pharisees and Sadducees (Matt. 16:4c). Seeking after signs is spiritual adultery (when committed by unbelievers) and spiritual fornication (when committed by believers). Listen to the apostle Paul's admonishment to the church:
Your boasting is not good. Do you not know that a little leaven leavens the whole lump? Clean out the old leaven, that you may be a new lump, just as you are in fact unleavened. For Christ our Passover also has been sacrificed. Let us therefore celebrate the feast, not with old leaven, nor with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth. 1 Corinthians 5:6-8
The apostle Paul also says "A little leaven leavens the whole lump" in Galatians 5:1-12, after asking the Galatians, "Who hindered you from obeying the truth?" We need to guard our doctrine (teaching) well, which Scripture admonishes us to do: "Pay close attention to yourself and to your doctrine (teaching)" (1 Timothy 4:16). People like to say, "doctrine divides." Yes, it surely does. It divides truth from lies and it divides genuine converts from false converts. "All Scripture is inspired by God and profitable for doctrine (teaching) [What is right.], for reproof [What is wrong.], for correction [How to get right.], for training in righteousness [How to stay right.]; that the man of God may be adequate, equipped for every good work" (2 Timothy 3:16-17). A little bit of false teaching slowly permeates through the congregation until the entire church is consumed by it. As examples, the Catholic church, the Charismatic church, and the Emergent Church and filled with nothing but leaven. However, this does not mean that the Reformed church or the Baptist church or other churches do not or cannot have leaven as well. We need wisdom, understanding, and discernment.