Yeast (or leaven) is a substance that people use in order to make bread. You only add a little yeast to the flour. The yeast grows and it spreads right through the flour. Jesus described this process in Matthew 13:33.
Here, Paul used yeast as a word picture of the effect of sin (evil deeds) in a Christianís life. When someone starts to do wrong things, it is difficult to stop. A Christian should deal with any sin that he may carry out, at once (1 John 1:9). Otherwise, that first act will lead to a series of evil actions. Such evil behaviour will certainly spoil that personís relationship with God. God hates all sin.
Unleavened bread is bread that does not contain yeast. People eat no other bread during the sacred holiday called the Passover. Because they are preparing the unleavened bread for a sacred purpose, they are very careful. They first remove anything that contains yeast from their houses. Then they clean the house. And only then can they make their bread.
That is how careful Christians should be to remove sin from their lives. They should examine every part of their own lives.
Paul links this with the idea of the Passover. Formerly, people killed a lamb (a young sheep) for the Passover meal. Exodus 12:21-23 explains why. It was so that God would save their families from death.
Like that lamb, Christ died during the Passover. John 1:29 explains why. It was so that God would save his people from sin and from death (Romans 6:23).
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© 2014, Keith Simons.