As used in the Bible, adultery generally refers to voluntary sexual intercourse by a married person with one of the opposite sex other than one’s mate, or, during the time that the Mosaic Law was in effect, such intercourse by any man with a married or a betrothed woman. The Hebrew root meaning “commit adultery” is na·ʼaphʹ, while its Greek counterpart is moi·kheuʹo.​—Eze 16:32, ftn; Mt 5:32, ftn.

Certain primitive societies allow free relations within the same tribe, but promiscuity outside tribal bounds is considered adultery. On the history of adultery, Funk & Wagnalls Standard Dictionary of Folklore, Mythology and Legend (1949, Vol. 1, p. 15) says: “It occurs in all parts of the world and though it is considered reprehensible by many cultures it has enjoyed a considerable popularity in all cultures and at all times.” Monuments attest to its prevalence in ancient Egypt; Potiphar’s wife, who proposed that Joseph have relations with her, was such an Egyptian. (Ge 39:7, 10) Historically as well as at present, adultery is generally forbidden, but penalties are seldom imposed.

Jehovah’s law separated Israel and raised the moral status of marriage and family life to a much higher level than that of the surrounding nations. The seventh commandment of the Decalogue stated in direct, unmistakable language: “You must not commit adultery.” (Ex 20:14; De 5:18; Lu 18:20) Adulterous invasion of another man’s domain was prohibited, as were other forms of sexual misconduct.​—See FORNICATION; PROSTITUTE.

Under the Law of Moses the penalty for adultery was severe​—death for both guilty parties: “In case a man is found lying down with a woman owned by an owner, both of them must then die together.” This applied even to a betrothed woman, it being considered that she had committed adultery if she had relations with a man other than the one to whom she was duly engaged. (De 22:22-24) If suspected of adultery, a wife had to stand trial.​—Nu 5:11-31; see THIGH.

Christians, though not under Mosaic Law, must also refrain from adultery. “For the law code, ‘You must not commit adultery,’ . . . is summed up in this word, namely, ‘You must love your neighbor as yourself.’” There can be no hypocrisy in this matter. (Ro 13:9; 2:22) In teaching Bible principles, Jesus raised the moral standard still higher for Christians. He broadened out the matter of adultery, saying it was not limited to sexual contact a man might have with a woman not his mate: “Everyone that keeps on looking at a woman so as to have a passion for her has already committed adultery with her in his heart.” Such men are among those who “have eyes full of adultery.”​—Mt 5:27, 28; 2Pe 2:14.

Jesus also pointed out that if a divorce was obtained by either husband or wife, except on the ground of fornication (Gr., por·neiʹa), the remarriage of either one would constitute adultery. Even a single man who took such a divorced  woman as his wife would be guilty of adultery.​—Mt 5:32; 19:9; Mr 10:11, 12; Lu 16:18; Ro 7:2, 3.

Adultery is “actually sin against God.” (Ge 39:9) Jehovah will judge those guilty of adultery, and none who persist in such a course “will inherit God’s kingdom.” (Mal 3:5; 1Co 6:9, 10; Heb 13:4) How true the proverb: “Anyone committing adultery with a woman is in want of heart; he that does it is bringing his own soul to ruin.”​—Pr 6:32-35.

How could one become guilty of spiritual adultery?

In a spiritual sense, adultery denotes unfaithfulness to Jehovah on the part of those who are joined to him in a covenant. Natural Israel in the Law covenant was, therefore, guilty of spiritual adultery because of false religious practices, some of which included sex-worship rites and disregard for the seventh commandment. (Jer 3:8, 9; 5:7, 8; 9:2; 13:27; 23:10; Ho 7:4) For similar reasons Jesus denounced as adulterous the generation of Jews in his day. (Mt 12:39; Mr 8:38) Likewise today, if Christians who are dedicated to Jehovah and who are in the new covenant defile themselves with the present system of things, they commit spiritual adultery.​—Jas 4:4.