Charles Ellicott Commentary 1 Corinthians 7:39-40

"A wife is bound for so long time as her husband liveth; but if the husband be dead, she is free to be married to whom she will; only in the Lord. But she is happier if she abide as she is, after my judgment: and I think that I also have the Spirit of God." — 1 Corinthians 7:39-40 (ASV)

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

The words “by the law” are not in the best manuscripts. The opening sentence, asserting the marriage union to be dissolvable only by death, is to guard against any married woman applying these words to herself, as they refer only to widows.

St. Paul explains that she is happier to continue as a widow, her case falling under the same considerations as those mentioned for the unmarried in the previous verses.

I think also that I have the Spirit of God.—This is not an expression of doubt about 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.