The celebration of the Feast of Unleavened Bread was an annual event, beginning the day after the Passover and lasting seven days, during which time no leaven (yeast) products were to be eaten. This commemorated their hasty exit (exodus) from Egypt (Ex 12:8,15,39, 13:6-10; Lk 22:1). Even today, when celebrating the Passover, Jewish houses are searched to ensure there is no yeast in them. Yeast, which quickly spreads throughout the dough, is a picture of sin so it reminds the Israelites about leaving the old, sinful life of Egypt and entering a new way of living (Lev 23:6-8; 1 Cor 5:7,8; Gal 5:9).
Jesus warned of the ‘yeast of the Pharisees’ – making the parallel between yeast and sin, stating only a little evil or wrong teaching can quickly spread, leading many people astray and bring into bondage (Mt 16:12).