In 1 Corinthians 5:7–8 the symbolism of leaven is gradually developed. Paul begins by comparing the sinner (the man discussed in 1 Corinthians 5:1–5) to leaven that must be cleansed out. By tolerating him and his sin, the church has allowed themselves to be defiled by him.
The danger is not so much that every church member will start committing the same sins as this man. Rather, the danger is that the church will become indifferent towards sin, thereby losing its purity as a people who have been redeemed by Christ’s blood. Paul reminds them of this very fact. Christ is the true Passover lamb (see also John 1:36; Revelation 5:1–14) who has been sacrificed for his people’s sin.
7 Cleanse out the old leaven that you may be a new lump, as you really are unleavened. For Christ, our Passover lamb, has been sacrificed.