1 Corinthians 7:39,40

The wife. — The question of the re-marriage of widows is here considered. It was probably a matter in which his opinion had been asked, and, in any case, naturally completes the subject of marriage. The widow may be married again if she desire, but “only in the Lord” — i.e., not to a heathen. She, being a Christian, should marry a Christian.

The words “by the law” are not in the best MSS. The opening sentence, asserting the marriage union to be dissoluble only by death, is to guard against any married woman applying these words to herself, they having reference only to widows.
St. Paul explains that she is happier to continue a widow (her case coming under the same considerations as referred to the unmarried in the previous verses).

I think also that I have the Spirit of God. — This is no expression of doubt as to whether he had the Spirit of God, but an assurance of his confidence that he, as well as other teachers (who, perhaps, boast more about it), had the Spirit of God to guide him in cases where no direct command has been given by Christ.