Adam Clarke on 1 Corinthians 7:12

But to the rest speak I, not the Lord - As if he had said: For what I have already spoken I have the testimony of the Lord by Moses, and of my own Lord and Master, Christ; but for the directions which I am now about to give there is no written testimony, and I deliver them now for the first time. These words do not intimate that the apostle was not now under the influences of the Divine Spirit; but, that there was nothing in the sacred writings which bore directly on this point.

If any brother - A Christian man, have a wife that believeth not, i.e. who is a heathen, not yet converted to the Christian faith, and she be pleased to dwell with him, notwithstanding his turning Christian since their marriage, let him not put her away because she still continues in her heathen superstition.