This means that the Apostle determines that such people be regarded as strangers to Christian society, that is, as excommunicated, like that incestuous man, the old leaven cast out.
The Apostle gives the reason for this: lest one be infected by them and leavened. Christians must be so holy and so faithful to the commandments that the thought should not even occur to them that one can sin and remain a Christian.
If manifest sinners remained in their community, then zeal for moral purity would be strongly shaken by the thought that Christians too can bear the name and not actually be what the name requires.
To prevent this general weakening, the Apostle commands them to cut off communion with such persons. St. Paul includes among them the fornicator, the covetous, the idolater, the reviler, the drunkard, and the robber.
Their very names give an understanding of them. A fornicator is one who openly commits fornication. The covetous man is one who grows rich by unjust trade, usury, or some deceit, and becomes unyielding, miserly, and grasping.
An idolater must be one who eats food sacrificed to idols without discernment, as St. Paul will explain later. A reviler is one who is abusive, quarrelsome, and given to gossip.
A drunkard is one who drinks immoderately and behaves disgracefully. A robber is a thief in various forms. Earlier he listed only the fornicator, the robber, the covetous, and the idolater, 1 Cor 5:10.
Here he adds the drunkard and the reviler, although these verses should correspond directly. One may conclude that even here not all are listed whom one must avoid or estrange from Christian society.
Therefore it is permissible to generalize the Apostle’s principle: every manifest sinner must be excluded from the Christian community. The Christian community must be pure.
As soon as anyone falls into sins, he must be cast out. In the earliest times sinners were in fact cast out.
Those who repented and sought communion again were received, yet they were subjected to strict trials. They had to pass through several stages of repentance in order to be received again into full communion.
This was the wise apostolic discipline. The multiplication of sinners made its fulfillment impossible. Try to fulfill it now, and you would have to drive everyone out. And there is no one to carry it out.