‘But to the married I give charge, yes, not I but the Lord, that the wife does not leave her husband, (but and if she does depart let her remain unmarried or else be reconciled to her husband), and that the husband does not leave his wife.'
Now he warns against misapprehension. He is not recommending divorce or separation so as to better serve God. It is God's direct command that a wife does not leave her husband, and a husband does not leave his wife. This is God's view of Christian marriage and He sees it as indissoluble. Thus they do God no service by disobeying Him. They are commanded to remain married.
‘Leave' here means leave permanently and with intent. It does not include such situations as Peter leaving his wife at home, to follow Jesus, but still acknowledging her as his wife (although it is, of course, possible that Peter's wife was one of the women who ministered to Jesus. Certainly she goes around with him in his later ministry - 1 Corinthians 9:5). There are times when such sacrifices are justifiable. But only if they do not lead into sin.
Paul, however, seems to accept that legitimate situations might arise where a woman can depart from her husband. This might be, for example, in cases of continual harshness, violence or insanity. Such situations can arise through great pressures or various illnesses which are not the fault of anyone. But in that case she must remain unmarried, with the alternative of returning and being reconciled with her husband if he becomes more amenable. That this is the possible scenario comes out in that there is no thought of a husband leaving a wife. He should be able to cope with his wife's violence. But this does not make her free to marry elsewhere. Marriage is binding for life.
Thus Paul is as firm as Jesus in stating that divorce is not permissible. However, as Jesus pointed out, the one thing that does permit divorce is when the other party is guilty of ‘fornication'. For that breaks the marriage bond because the person has now become linked to an adulterer. Under the Law indeed they should be put to death (Deuteronomy 22:22), and the innocent party would then be free to marry again. Thus in view of the relaxation of that Law the same outcome is considered to arise. The guilty party is ‘seen as dead'.
Alternately in the case of such women Paul may be providing for cases of leaving the husband in straight disobedience to God's command, although if that were so it is difficult to see Paul accepting it so placidly, and if it is so why not vice versa as well? Even if he has a particular case in mind why does he not command a reconciliation? By his statements she is guilty of disobedience to God. Thus the ‘leaving' is possibly rather seen as due to necessity for one reason or another, something so severe that it justifies leaving. He is not speaking of just walking out due to personal preference.
In view of the stated purpose of marriage in the whole passage it is difficult to think of any other grounds for desertion which would be acceptable to Paul, especially in view of his statement in 1 Corinthians 7:3-4 and his other injunctions in this verse. Presumably ‘remaining unmarried' here means not seeking to obtain a divorce in order to remarry. But his main point is that a woman who has left her husband is not free to marry another while he lives. Marriage is inviolable unless destroyed by sexual misbehaviour which breaks the marriage bond.
‘Not I but the Lord.' It is probable that this is put in for special emphasis because this issue was especially alive and pressing and one in which some were saying, ‘it is only Paul who is saying that. We have been inspired to see it differently'. The issue was so huge that he wanted it to be quite clear that the authority behind his words was the maximum possible. This was not just the words of one ‘prophet' as against another, or even of an Apostle, they were the words of Jesus Himself. Thus Paul is saying, ‘take especial note that this is not just my command, it is the Lord's.' The inviolability of marriage was primary and was directly Jesus' commandment. Nothing could circumvent that. Once and for all the issue was decided.