It is sometimes said that leaven always represents sin in the Bible. But it is unwise to make such sweeping statements, since each case should be evaluated on its own. Jesus used leaven as a metaphor to describe the teaching and hypocrisy of the Pharisees, Sadducees, and Herod (Matthew 16:6, 11, 12, Mark 8:15, and Luke 12:1), which are clearly allusions to sin.
On the other hand, He used leaven in a parable, to describe how the Kingdom of Heaven would spread throughout the world – like leaven in dough. Should we believe He is suggesting that the spreading aspect of the Kingdom is sin? At the Feast of Weeks (aka Pentecost), the worshippers were to bring two loaves of leavened bread to the priest as a wave offering. Should we believe that their offering represented some sort of sin that they were bringing before the Lord in this joyous feast? In both of these examples it is difficult to believe the leaven represents sin.
In all of the above passages, we should realize that leaven is being used figuratively, but in different ways. It can be used to represent good things or bad things, and it is up to us to determine the writer or speaker’s intended meaning in each case. As a general rule of interpretation, we cannot assume that the meaning of a figure of speech carries the same meaning from one passage to another. We must evaluate each passage in its own context and then determine the meaning of leaven, or of whichever figure of speech is being used.