Albert Barnes Commentary 1 Corinthians 7:39

"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." — 1 Corinthians 7:39 (ASV)

The wife is bound, and so on. (See Barnes on Romans 7:2).

Only in the Lord. That is, only to one who is a Christian; with a proper sense of her obligations to Christ, and so as to promote his glory. The apostle supposed that could not be done if she were allowed to marry a heathen, or one of a different religion. He advances the same sentiment in 2 Corinthians 6:14; and it was his intention, undoubtedly, to affirm that it was proper for a widow to marry no one who was not a Christian. The reasons at that time would be obvious.

1. They could have no sympathy and fellow-feeling on the most important of all subjects if the one was a Christian and the other a heathen. (See 2 Corinthians 6:14–15 and following).
2. If she should marry a heathen, would it not be showing that she had not as deep a conviction of the importance and truth of her religion as she ought to have? If Christians were required to be separate, to be a peculiar people, not to be conformed to the world, how could these precepts be obeyed if the society of a heathen was voluntarily chosen, and if she became united to him for life?
3. She would in this way greatly hinder her usefulness; put herself in the control of one who had no respect for her religion, and who would demand her time and attention, and thus interfere with her attendance on the public and private duties of religion and the offices of Christian charity.
4. She would thus greatly endanger her piety. There would be danger from the opposition, the taunts, and the sneers of the enemy of Christ; from the secret influence of living with a man who had no respect for God; from his introducing her into society that was irreligious and that would tend to mar the beauty of her piety and to draw her away from simple-hearted devotion to Jesus Christ.

And do not these reasons apply to similar cases now? And if so, is not the law still binding? Do not such unions now, as really as they did then, place the Christian where there is no mutual sympathy on the subject dearest to the Christian heart? Do they not show that she who forms such a union has not as deep a sense of the importance of piety, and of the pure and holy nature of her religion, as she ought to have?

Do they not take time from God and from charity? Do they not break up plans of usefulness and lead away from the society of Christians and from the duties of religion? Do they not often expose to ridicule, to reproach, to persecution, to contempt, and to pain? Do they not often lead into society, by a desire to please the partner in life, where there is no religion, where God is excluded, where the name of Christ is never heard, and where the piety is marred and the beauty of simple Christian piety is dimmed?

And if so, are not such marriages contrary to the law of Christ? I confess that this verse, to my view, proves that all such marriages are a violation of the New Testament; and if they are, they should not on any plea be entered into. It will be found, in perhaps nearly all instances, that they are disastrous to the piety of the married Christian, the occasion of ultimate regret, and the cause of a loss of comfort, peace, and usefulness in the married life.