To understand this verse of Scripture, it is helpful to understand the process of baking bread in the first century. In Paul’s day, when a woman would bake bread she would get all the dough ready and then would put it into whatever receptacle it was to be baked in. She would then take a piece of dough out and roll it into a ball and place it in water. Over time, this piece of dough would then sour and would be taken out and set aside until it was time to bake more bread. When the time came, she would put the old piece of dough into the new dough as a starter. This small piece of soured dough would greatly influence the entire new loaf affecting both the taste and the look of the bread.
Leaven is one of the most commonly used idioms in the Scriptures. As you can see from the description above, it carries with it the idea of something from the past influencing something in the present. So Paul uses this idiom here in 1 Corinthians 5 because he wants his readers to understand that in the same way a little piece of soured dough from the old influences an entire new lough of bread, so a little sin from ones old life influences and corrupts ones new life in Christ.
In v. 7 Paul calls for his readers to, “cleanse out the old leaven.” He says, “Take it out, purge it out.” In other words, there should be nothing from your old life mixed in with the new.
