I Corinthians 7:12: "But I (not the Lord)..."
I studied this years ago, and I realize Paul is capping his statement from verse ten. But Why? Is this not from the Lord?
In I Corinthians 7:10-11, Paul addresses a command that is based on the Lord’s authority, given while Jesus was here on earth. While Paul doesn't directly quote Jesus, he is referring to Jesus' teaching on marriage and divorce, which is found in Matthew 19:1-12; Mark 10:2-12; and Luke 16:18. Paul uses a Greek idiom "not ... but" where a common verb ("give") is used between the two phrases. This idiom does not have the same meaning in English as it has in English. We use "not ... but" to indicate exclusiveness. The Greeks used this phrasing to indicate that the "not" portion was not nearly as important as the "but" portion. Thus, Paul is not saying that he doesn't teach that a wife should not leave her husband. He is saying that he does teach this, but it is not nearly as important as the fact that Jesus taught this while he was here on earth.
I Corinthians 7:12-16, Paul addresses a topic that Jesus did not mention while he was here on earth. The topic is what happens in marriages where only one spouse is a believer. Thus, Paul said, "to the rest" meaning those in mixed marriages. It is a topic he has taught on, but the Lord did not mention it while he was on earth.
This does not mean that Paul is teaching something contrary to what the Lord wanted taught. The Lord had told the apostles that he had not covered everything. "I have many more things to say to you, but you cannot bear them now. But when He, the Spirit of truth, comes, He will guide you into all the truth; for He will not speak on His own initiative, but whatever He hears, He will speak; and He will disclose to you what is to come. He will glorify Me, for He will take of Mine and will disclose it to you" (John 16:12-15). Paul is disclosing one of those topics.