The wife is bound by the law as long as her husband lives; but if her husband is dead, she is at liberty to be married to whom she will; only in the Lord.
All Commentaries on 1 Corinthians 7:39 Go To 1 Corinthians 7
Ambrosiaster
AD 400
Paul writes this in order to make it clear that a woman who has been rejected by her husband is not free to marry again. If he should die, then she may remarry, but only in the Lord, which means without any suspicion of wrongdoing and within the bounds of the church. Commentary on Paul’s Epistles.