Paul asks for the affection of the Corinthians, complaining that he has affection for them, but it is not returned. Verses 6:14 thru 7:1 seem to break the flow of this argument, which resumes at 7:2, and presenting directions on mixed marriages that are not entirely compatible with his more detailed instructions in 1 Corinthians.  We had some sympathy with the commentators who believe that this section is a fragment that belongs to another letter, and has been edited in by a later copyist.  Paul refers to a "stern letter", and indeed apologizes for it ("I did not mean to make you grieve"). This letter does not appear to be I Corinthians. However, the beginning of chapter 10 is a very abrupt change of tone, and it has been suggested that this, too, belongs to another letter, perhaps the "stern letter" itself.

Another oddity is that Paul twice refers to a "brother" whom he is sending to Corinth, without giving his name.  Some have suggested that the name appeared in the original manuscript, but was removed when that person developed a bad reputation.  "Demas" who "deserted me and went to Thessalonica" (2 Tim 4:10) is one candidate.  In Chapter 10 (verses 4-6), Paul surprisingly threatens the Corinthians with supernatural "weapons" that he has at his disposal.