In the past when making bread, a piece of dough, from a former batch (that had fermented and turned acid), was added to the other ingredients to cause it to lighten or rise due to the trapped air. Nowadays this is often replaced by yeast that has the same effect. Only a small amount of a fermenting or rising agent (for bread or liquids) is required for its effect to be seen as, given time, its influence permeates throughout (Lk 13:21; Gal 5:9).
During the Passover no leaven was allowed to be used or even present in the Israelite houses (Ex 23:18, 34:25). This was to remind people of their hasty exodus from Egypt (Deut 16:3).
Jesus warned about the leaven of the Pharisees and Sadducees (Mt 16:6-12; Lk 12:1). Figuratively, this illustrated their personal corruption and the corporate, defiling effect it was having on society – through their hypocritical double standards and manmade rules or traditions (Mt 23:2-33).
Bad habits are like yeast – quickly affecting our whole character
have a detrimental effect and monumental outcome if left unaddressed. Because of its potential impact, when we are aware of sin ruthless action should be taken to eliminate it from our attitudes, thoughts, speech and actions, replacing them with Godly qualities (Gal 5:16-24; Eph 4:22-32; Phil 4:8; Col 3:8,9).
Although leaven is usually a symbol of evil and corruption in the Bible, Jesus also compared it with the gospels ability to permeate and
bring about positive change in society (Mt 13:33). The gospel has the capability to penetrate and the power to transform our whole being,
which should affect those around us. How effective is the gospel in bringing change within me?