Yeast is the leaven in bread.
Before the Passover evening, the Israelites were commanded to remove all the leaven or yeast from their houses. During the Passover meal, they were to eat unleavened bread.
WHY?
Anciently, yeast was viewed as a corruptible agent. Although yeast makes bread rise, it also causes fermentation. Perhaps it was Jehovah’s way of teaching Israel to remove all corruption from their lives.
It may have also represented man’s inability to rise from corruption and put on incorruption without the gospel and without the Savior. Without the atonement, no man can put on incorruption for resurrection is not possible without Jesus Christ.