“It is not right for you to be proud! You know the saying, ‘A little bit of yeast makes the whole batch of dough rise’. You must remove the old yeast of sin so that you will be entirely pure. Then you will be like a new batch of dough without any yeast”. 
1 Cor 5:6, 7 GNB 

Today in many countries where Christianity has been practiced for centuries there is not the clear cut distinction between the Christian community and the surrounding society. But in countries where Christianity is not the norm, the lines are much sharper. 
They were clear in the ancient world because Christians were a tiny minority and the faith was a newcomer on the scene. Furthermore the behavioural standards and discipline of Christians set them in sharp contrast to the rest of the community. They drew hostility because of this. So Paul, having been informed of the shameful sexual scandal that was going on in Corinth, tried to make them see that they had no cause to be proud when such goings-on were happening. He was also aware that, if unchecked, the practice could lead to other unhealthy things as well. His approach was simple. “Nip it in the bud straight away. Don’t let it spread or contaminate the rest of the Christian fellowship”. It was probably the kind of thing that went on in the surrounding society of Corinth, notorious as it was for its filth. Christians had to be different. And they had to be seen to be different. There were to be no half measures. 
Let it be so in your Christian community as well. Know well that gross sin will corrupt the witness to the gospel more than anything else. Keep the standards of Christ. 

Lord, help us to guard the standards of Jesus Christ.