For to the rest, &c. This was a case entirely new, which the wisdom of the apostle regulates according to the laws of charity. Tertullian thinks that some of the faithful, who had been converted from paganism, did not esteem it lawful to live any longer with their wives, who were yet buried in the superstitions of idolatry, which scruples St. Paul answers, guided as he was, by the particular lights of the Holy Ghost. (Calmet)

Not the Lord. That is, it is the command of the Lord, for such even as are separated, not to marry to another, but when I advised the unmarried not to marry, this is a counsel, or advice, not a divine precept, which doctrine he repeats again before the end of this chapter, ver. 25, 28, and 39.

If any brother have a wife that believeth not, &c. St. Paul speaks of two that were joined by a contract of marriage, when both of them were infidels, and that one of them is converted to the Christian faith: we do not read of any precept that Christ gave, as to those marriages, but the apostle seems to order by his apostolical authority, that they continue as man and wife, unless the party that remains still an infidel, will needs depart; then, says the apostle, let such an one depart. There is also another case, to wit, when the man or woman remaining an infidel, will not live without continual injuries and blasphemies against God and the Catholic religion, so that there can be no peace on that account betwixt them. In these two cases, according to the canons of the Church, it is looked upon as no marriage, so that the party converted may marry another. And this seems grounded on the reason, which the apostle here gave, that God hath called us in peace. (Witham)