1 Corinthians 5:7a
7 Purge out therefore the old leaven, that ye may be a new lump, as ye are unleavened.
The old leaven he is figuratively using concerns boasting over sin by the church members. That is what they need to purge. Paul isn’t referring to the man involved in fornication. He already gave the command on what should be done with him (5:5).
His attention is focused on the pressing problem of the carnality among the members. They need to purge themselves of pride caused by their boasting. They need to be a new lump of dough or a fresh batch of dough–start fresh! The church is not to be puffed up; instead, it is to be unleavened.