Was the sin in the Corinthian church public or hidden?
The sin in the Corinthian church was public, not hidden. Paul writes that it was reported commonly and was not done in a corner. The whole congregation knew about it, and even the pagan Gentiles would not commit such immorality.
Les explains that 1 Corinthians chapter 5 is not a pretty chapter and he would just as soon not have to comment on it, but since they are going chapter by chapter, he must deal with it. Paul writes to the same congregation, and the sin is reported commonly—not something done in a corner, but open. The whole Corinthian congregation, whether large or small, knew about him. The immorality was fornication, such as is not even named among the Gentiles; even those pagan Gentiles would not do what this couple was doing, namely, that one has his father's wife.
It is reported commonly that there is fornication among you, and such fornication as is not so much as named among the Gentiles, that one should have his father’s wife.