Calvin's Commentaries, Vol. 5: Harmony of the Law, Part III, tr. by John King, [1847-50], at sacred-texts.com
14. Thou shalt not commit adultery.
THE REPETITION OF THE COMMANDMENT
18. Neither shalt thou commit adultery.
Although one kind of impurity is alone referred to, it is sufficiently plain, from the principle laid down, that believers are generally exhorted to chastity; for, if the Law be a perfect rule of holy living, it would be more than absurd to give a license for fornication, adultery alone being excepted. Furthermore, it is incontrovertible that God will by no means approve or excuse before this tribunal, what the common sense of mankind declares to be obscene; for, although lewdness has everywhere been rampant in every age, still the opinion could never be utterly extinguished, that fornication is a scandal and a sin. Unquestionably what Paul teaches has been prevalently received from the beginning, that a good life consists of three parts, soberness, righteousness, and godliness, (Titus 2:12;) and the soberness which he commands differs not from chastity. Besides, when Christ or the Apostles are treating of a perfect life, they always refer believers to the Law; for, as it had been said of old by Moses, “This is the way, walk ye in it;” 59 Christ confirms this,
“If thou wilt enter into life, keep the commandments,” (Mt 19:17;)
and Paul corroborates it, “He that loveth another hath fulfilled the Law,” (Ro 13:8,) whilst they constantly pronounce a curse against all fornicators. It is not worth while to quote the particular passages in which they do so. Now, if Christ and the Apostles, who are the best interpreters of the Law, declare that God’s Law is violated no less by fornication than by theft, we assuredly infer, that in this Commandment the whole genus is comprehended under a single species. Wherefore, those have done nothing but betray their disgraceful ignorance, who have sought to be praised for their acuteness on the score of their ridiculous subtlety, when they admitted that fornication is indeed condemned with sufficient clearness and frequency in the New Testament, but not in the Law. For, if they had reasoned justly, inasmuch as God is declared to have blessed marriage, it must at once be concluded, on the contrary, that the connection of male and female, except in marriage, is accursed. This is the argument of the author of the Epistle to the Hebrews, where he contrasts two opposite things;
“Marriage (he says) is honorable in all, and the bed undefiled; but whoremongers and adulterers God will judge.”
(Heb 13:4.)
So also, when God forbids the priest to marry a harlot, (Le 21:14,) the manifest impropriety of fornication is declared; and, if it was unlawful for the daughters of Israel to be harlots, (De 23:17,) the same reasoning applies necessarily to males. Nor has Hosea taken that reproof from anywhere else but the Law? “Whoredom and wine take away the heart.” (Ho 4:11.) Thus, when the Prophets metaphorically condemn the corruptions of their nation, they do not always use the same; word as Moses here does, נפ, naaph, but compare them to fornications, whereas, if fornication were lawful in itself, this metaphor would be altogether inappropriate. Hosea was commanded to take a harlot for a wife, (Ho 1:2;) no mention is made of adultery, and still the shame and baseness of the people is thus condemned. Who, then, would say that fornication is free from sin, since God brands it with no ordinary mark of ignominy? But if any should pertinaciously contest this, let him accuse Paul of error, who bears witness that an example is set before us in the Law, that we should. not “commit fornication as some of them committed, and fell in one day three-and-twenty thousand.” (Nu 25:9; 1Co 10:8.) Surely, if they had not transgressed the Law, so horrible a vengeance would not have overwhelmed them. If any should object that the crime of idolatry was mixed up with it., still the declaration of Paul remains untouched, that God was the avenger of fornication in this infliction of punishment, which would not accord, unless it were a transgression of the Law. And in truth, where, as recorded by Luke, (Ac 15:20,) the Apostles in their decree prohibit fornication amongst the Gentiles, the reason is at the same time added, that “Moses is read in the synagogues.” Now, if it were not a vice opposed to the Law, no offense would have hence arisen.
We have already explained why, under this word adultery, every impure lust was condemned. We know how unbridled was the licentiousness of the Gentiles; for, although God never suffered all shame to be extinguished together with their purity, still respect for what was right was in a manner stifled, so that they evaded the grossness of the sin by ribaldry and scurrilous jests. At any rate, the doctrine of Paul was by no means understood, that those who indulge in whoredom “sin against their own body.” (1Co 6:18.)
Since, then, the minds of all men were stupified by indulgence, it was needful to arouse them by declaring the atrocity of the sin, that they might learn to beware of all pollution. Nor are unbridled lusts only here condemned, but God instructs His people to cherish modesty and chastity. The sum is, that those who desire to approve themselves to God, should be pure “from all filthiness of the flesh and spirit,” (2Co 7:1;) nor can we doubt but that Paul in these words would interpret the law, as he elsewhere exhorts,
“that everyone should possess his vessel in sanctification and honor; not in the lust of concupiscence, even as the Gentiles which know not God.” (1 Thess. 4:4, 5.)
The quotation is not from the writings of Moses, but an accommodation from Isa 30:21.