Sin can be a pervasive thing in the church. Like yeast works its way through a whole lump of dough, sin can work its way through a church body, leaving terrible consequences. Habitual sin in a believer's life needs to be confronted and dealt with for the benefit of the believer and for the safety of the church.
"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." 1 Corinthians 5:6-7