Your glorying is not good. Do you not know that a little leaven leavens the whole lump?
— 1 Corinthians 5:6
As believers, we are interconnected. The sin of one will affect many. That is why the apostle Paul said the church should never tolerate evil. He said, "And if one member suffers, all the members suffer with it; or if one member is honored, all the members rejoice with it" (1 Corinthians 12:26).
Apparently in the Corinthian church, there was a man who was sleeping his father's wife (not his biological mother but a woman his father had married). The church was actually boasting about how liberal and tolerant they were. So Paul confronted them, saying, "And you are puffed up, and have not rather mourned, that he who has done this deed might be taken away from among you" (1 Corinthians 5:2).
If an unbeliever who is living an immoral lifestyle comes to our church, we'll welcome that person. We'll say, "We love you." We'll also say, "Jesus Christ wants to change your life." We will call him or her to the Lord and to faith.
But if a Christian comes to our church and is living openly in sin, if we find out about it, we will call him or her to repentance. But if that Christian refuses to repent, then he or she will be asked to leave.
Some might think that isn't very loving. But actually it is very loving, and I'll tell you why. If believers are living openly in sin, and the church doesn't do anything about it, it's sending a message that everything is okay and that we can thumb our noses at God.
Paul said, "Do you not know that a little leaven leavens the whole lump?" (1 Corinthians 5:6). In modern vernacular, a little yeast permeates the whole batch of dough. If sin is tolerated, it will spread and corrupt others.