1 Cor 5:1-5
"It is actually reported that there is sexual immorality among you, and of a kind that is not tolerated even among pagans, for a man has his father's wife. And you are arrogant! Ought you not rather to mourn? Let him who has done this be removed from among you."
The case: a man living with his father's wife (stepmother)—forbidden both by Roman law and Mosaic law (Lev 18:8). The Church not only tolerated it but was proud of it (likely as a demonstration of 'Christian liberty'). Paul commands the exclusion of the individual for the 'good' of his soul—the discipline has a restorative purpose.
1 Cor 5:6-8
"Your boasting is not good. Do you not know that a little leaven leavens the whole lump? 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."
The metaphor of leaven (Prov 25:4; Gal 5:9): tolerated sin contaminates the entire community. The reference to Passover is profound: Christ is the Passover Lamb sacrificed; the Church must be 'unleavened' (unleavened bread)—a purity flowing from Christ's work, not human effort.