John Calvin Commentary 1 Corinthians 7:15

"Yet if the unbelieving departeth, let him depart: the brother or the sister is not under bondage in such [cases]: but God hath called us in peace." — 1 Corinthians 7:15 (ASV)

But if an unbeliever depart. This is the second part of his statement, in which he sets free a believing husband who is prepared to live with an unbelieving wife but is rejected by her, and in the same way a woman who is, through no fault of her own, rejected by her husband. For in that case, the unbelieving party is divorcing God rather than his or her partner. Therefore, there is a special reason in this case, since the first and chief bond is not merely loosened but even completely broken. While some believe that we are today in a very similar situation with Papists, we should wisely consider what difference exists between the two cases, so that we do not attempt anything rashly.

In peace. Here also, interpreters differ, for some understand it this way — “We are called in peace; therefore, let us avoid all basis and occasion for quarrels.” I understand it more simply: “Let us, as much as we can, cultivate peace with everyone, to which we have been called. Therefore, we must not rashly separate from unbelievers unless they initiate the divorce. God, therefore, has called us in peace for this purpose, so that we might cultivate peace with everyone by acting properly toward everyone.” This, then, belongs to the first part of his statement—that believers should remain with unbelievers if they are willing to live with them (1 Corinthians 7:12–13), because a desire for divorce is inconsistent with our profession.