I had always just assumed that Paul was merely giving his opinion in I Corinthians when he says “But to the rest I say, not the Lord, that if any brother has a wife who is an unbeliever, and she consents to live with him, he must not divorce her”. It just seems obvious that Paul is giving his own personal opinion and not saying anything authoritative in that verse. So as such, we (and the Corinthians) can take it or leave it.
I recently read this article and it’s totally changed my mind about the authoritative claims in this passage. Paul it seems is writing the whole letter from his own apostolic authority as the founder of the church in Corinth. When he says in an earlier verse that “not I, but the Lord says. . .” he is referring to the teachings of Jesus on marriage which had not been written down into the Gospels yet. Paul is actually giving us the earliest written portion on the Sermon on the Mount, which to this point had been passed on in the oral tradition.
So it’s not that Paul is doing some sort of automatic writing as he pens this letter with God whispering in his ear the whole time, and then Paul hears silence and decides to let us know he’s throwing his own thoughts in. The entire letter is formed by Paul’s own thoughts and he pauses to tell us something Jesus specifically said. Then he lets us know that he’s back to offer more of his own authoritative instruction.