In addition to Matthew 18, there are other passages that talk about church discipline. In 3 John, for example, the apostle John is writing about a man named Diotrephes, and he publicly calls attention to his bad behavior. Nothing else is said about it, but it seems to have been a situation that required some kind of official church action. In 1 Timothy 1 Paul mentions a case involving two men whom he “handed over to Satan to be taught not to blaspheme” (v. 20). We do not know anything more about it than that, but Paul does use the same kind of language here in 1 Corinthians 5:5. In addition, some might put forward the example of Ananias and Sapphira in Acts 5. Although this is a case of divine judgment for their sin, rather than that of discipline exercised through the local church, still we can say that it produced similar effects with regard to the church as a whole. What these examples show is that what we find in 1 Corinthians 5 is not an isolated passage that we can brush aside as only being relevant in Paul’s day. Discipline was practiced in the New Testament churches, and it still needs to be practiced today in order to protect the honor of Christ and to seek to bring the wayward member to repentance and restoration.
Paul begins chapter 5 by describing the offense. First of all, we notice that it was a significant transgression. What it actually involved was a sexual sin in which, as Paul says, a man has his father’s wife. That Paul does not use “mother” to describe the woman is generally taken to mean that the man was living with his stepmother. It does not say whether his father was living, or divorced, or what the particular circumstances were. The important thing is that it is an offense against the law of God and, as Paul says in the passage, even against the proper understanding of morality among the pagans.
Concerning the law of God, it is a violation of the Ten Commandments, specifically the commandment against adultery, which is rightly understood to include a variety of sexual offenses. It is explicitly a transgression of the laws concerning the control of marriage between related persons that you find in Leviticus 18. Thus, on the Jewish and Christian side based upon the Old Testament Scriptures, it was an offense. But it was also an offense in the mind of the pagans of the day. Paul says that it is such a thing that even the pagans would not tolerate.
The second thing he talks about is the fact that it was public. It was not even something that had happened in a quiet way, which you might perhaps say could be dealt with privately. There is a good principle there that if a private wrong can be made right quietly without making it known to a broader group, that is certainly the procedure to be followed. But in this case that was not possible, because this was something that, apparently, was well known in Corinth. People said, “Oh, Christians are supposed to be followers of Jesus Christ and to have this high standard of morality. Well, you know what’s going on down there? They’re doing the sort of things that we wouldn’t even tolerate here in Corinth. And everybody knows how bad Corinth is.” The problem had even made its way out of Corinth because Paul heard about it, perhaps in Ephesus.
The third point is that it was having an evil effect upon the church. Paul seems indeed to be more concerned with that than with the offender. He in effect says to the church, “Look, the problem here is that this public, obvious scandal exists. And not only are you not offended by it and refuse to mourn for this obvious sin in your midst, but you’re even proud about it.” How could they boast about it? It is hard to imagine a Christian doing that. Paul does not explain, but I suppose it could mean that they were proud of what we would call their tolerance or their broadmindedness where moral matters are concerned.

