43 Here there arises a second question, when the Lord allows a wife to be put away for the cause of fornication, in what latitude of meaning fornication is to be understood in this passage—whether in the sense understood by all, viz. that we are to understand that fornication to be meant which is committed in acts of uncleanness; or whether, in accordance with the usage of Scripture in speaking of fornication (as has been mentioned above), as meaning all unlawful corruption, such as idolatry or covetousness, and therefore, of course, every transgression of the law on account of the unlawful lust [involved in it]. But let us consult the apostle, that we may not say rashly. “And unto the married I command,” says he, “yet not I, but the Lord, Let not the wife depart from her husband: but and if she depart, let her remain unmarried, or be reconciled to her husband.” For it may happen that she departs for that cause for which the Lord gives permission to do so. Or, if a woman is at liberty to put away her husband for other causes besides that of fornication, and the husband is not at liberty, what answer shall we give respecting this statement which he has made afterwards, “And let not the husband put away his wife”? Wherefore did he not add, saving for the cause of fornication, which the Lord permits, unless because he wishes a similar rule to be understood, that if he shall put away his wife (which he is permitted to do for the cause of fornication), he is to remain without a wife, or be reconciled to his wife? For it would not be a bad thing for a husband to be reconciled to such a woman as that to whom, when nobody had dared to stone her, the Lord said, “Go, and sin no more.” And for this reason also, because He who says, It is not lawful to put away one's wife saving for the cause of fornication, forces him to retain his wife, if there should be no cause of fornication: but if there should be, He does not force him to put her away, but permits him, just as when it is said, Let it not be lawful for a woman to marry another, unless her husband be dead; if she shall marry before the death of her husband, she is guilty; if she shall not marry after the death of her husband, she is not guilty, for she is not commanded to marry, but merely permitted. If, therefore, there is a like rule in the said law of marriage between man and woman, to such an extent that not merely of the woman has the same apostle said, “The wife has not power of her own body, but the husband;” but he has not been silent respecting him, saying, “And likewise also the husband has not power of his own body, but the wife;”— if, then, the rule is similar, there is no necessity for understanding that it is lawful for a woman to put away her husband, saving for the cause of fornication, as is the case also with the husband.
44. It is therefore to be considered in what latitude of meaning we ought to understand the word fornication, and the apostle is to be consulted, as we were beginning to do. For he goes on to say, “But to the rest speak I, not the Lord.” Here, first, we must see who are “the rest,” for he was speaking before on the part of the Lord to those who are married, but now, as from himself, he speaks to “the rest:” hence perhaps to the unmarried, but this does not follow. For thus he continues: “If any brother has a wife that believes not, and she be pleased to dwell with him, let him not put her away.” Hence, even now he is speaking to those who are married. What, then, is his object in saying “to the rest,” unless that he was speaking before to those who were so united, that they were alike as to their faith in Christ; but that now he is speaking to “the rest,” i.e. to those who are so united, that they are not both believers? But what does he say to them? “If any brother has a wife that believes not, and she be pleased to dwell with him, let him not put her away. And the woman which has an husband that believes not, and if he be pleased to dwell with her, let her not put him away.” If, therefore, he does not give a command as from the Lord, but advises as from himself, then this good result springs from it, that if any one act otherwise, he is not a transgressor of a command, just as he says a little after respecting virgins, that he has no command of the Lord, but that he gives his advice; and he so praises virginity, that whoever will may avail himself of it; yet if he shall not do so, he may not be judged to have acted contrary to a command. For there is one thing which is commanded, another respecting which advice is given, another still which is allowed. A wife is commanded not to depart from her husband; and if she depart, to remain unmarried, or to be reconciled to her husband: therefore it is not allowable for her to act otherwise. But a believing husband is advised, if he has an unbelieving wife who is pleased to dwell with him, not to put her away: therefore it is allowable also to put her away, because it is no command of the Lord that he should not put her away, but an advice of the apostle: just as a virgin is advised not to marry; but if she shall marry, she will not indeed adhere to the advice, but she will not act in opposition to a command. Allowance is given when it is said, “But I speak this by permission, and not of commandment.” And therefore, if it is allowable that an unbelieving wife should be put away, although it is better not to put her away, and yet not allowable, according to the commandment of the Lord, that a wife should be put away, saving for the cause of fornication, [then] unbelief itself also is fornication.
45. For what do you say, O apostle? Surely, that a believing husband who has an unbelieving wife pleased to dwell with him is not to put her away? Just so, says he. When, therefore, the Lord also gives this command, that a man should not put away his wife, saving for the cause of fornication, why do you say here, “I speak, not the Lord”? For this reason, viz. that the idolatry which unbelievers follow, and every other noxious superstition, is fornication. Now, the Lord permitted a wife to be put away for the cause of fornication; but in permitting, He did not command it: He gave opportunity to the apostle for advising that whoever wished should not put away an unbelieving wife, in order that, perchance, in this way she might become a believer. “For,” says he, “the unbelieving husband is sanctified in the wife, and the unbelieving wife is sanctified in the brother.” I suppose it had already occurred that some wives were embracing the faith by means of their believing husbands, and husbands by means of their believing wives; and although not mentioning names, he yet urged his case by examples, in order to strengthen his counsel. Then he goes on to say, “Else were your children unclean; but now are they holy.” For now the children were Christians, who were sanctified at the instance of one of the parents, or with the consent of both; which would not take place unless the marriage were broken up by one of the parties becoming a believer, and unless the unbelief of the spouse were borne with so far as to give an opportunity of believing. This, therefore, is the counsel of Him whom I regard as having spoken the words, “Whatsoever you spend more, when I come again, I will repay you.”
Source: Our Lord's Sermon on the Mount (New Advent)