Yeast is used symbolically in the Bible to represent influence, often negatively. Just as a little bit of yeast, permeates dough and produces a noticeable result, so does influence, be it good or bad.
Another negative connotation is when Jesus says to beware of the yeast of the Pharisees, which is hypocrisy (Luke 12:1), as well as the yeast of Sadducees (Matthew 16:6-12) and the yeast of Herod (Mark 8:14-15).
Another reference to yeast is in 1 Corinthians 5:6-8, where Paul uses yeast as a metaphor for boasting. Paul says to get rid of bread with old yeast (malice and wickedness) and use new bread without yeast (sincerity and truth).
Key verse about Yeast: He told them still another parable: “The kingdom of heaven is like yeast that a woman took and mixed into about sixty pounds of flour until it worked all through the dough.”(Matthew 13:33, NIV)
A lifelong student of the Bible, Peter DeHaan, PhD, wrote the 700-page website ABibleADay.com to encourage people to explore the Bible. His main blog and numerous books urge Christians to push past the status quo and reconsider how they practice their faith in every area of their lives.