Yeast is used symbolically in the Bible to represent influence, usually negatively.^[ _citation\ needed_]^ Just as a little bit of yeast, works through and permeates dough, thereby producing a noticeable result (that is, the bread rises), so to does influence, be it good or bad.
In the Books of Moses, there are 32 references to not using yeast in various religious practices. Symbolically, the removal of yeast from the bread shows the removal of sin from our life.^[ _citation\ needed_]^
This theme continues in Judges 6:19, I Samuel 28:24, and Ezekiel 45:21.
(Interestingly, Leviticus 7:13 and 23:17 are two exceptions to this them of no yeast, when yeast was in fact prescribed.)
Paul says that false teaching is like yeast (Galatians 5:9).
Another negative connotation appears in Matthew 16:6-12, Mark 8:14-15, and Luke 12:1 where Jesus warns to beware of the yeast of the Pharisees (which Luke likens to hypocrisy) and also the Sadducees (per Matthew) and Herod (per Mark).
In seemingly the only positive usage of yeast in the Bible, Jesus says that the Kingdom of God / Kingdom of Heaven is like yeast (Matthew 13:33 and Luke 13:18-21).
A final reference to yeast is in 1 Corinthians 5:6-8, where Paul uses yeast as a metaphor for boasting. Paul instructs that we are to get rid of bread with old yeast (representing malice and wickedness) and to use new bread without yeast (which represents sincerity and truth).