The word adultery is etymologically related to the word adulterate, which means “to render something poorer in quality by adding another substance.” Adultery is the adulteration of marriage by the addition of a third person. Adultery is voluntary sexual activity between a married person and someone other than his or her spouse.
The Bible begins its teaching on marriage with the pattern of Adam and Eve: one man and one woman, husband and wife, united by God (Genesis 2:24, Mark 10:7–9). Adultery is forbidden by the seventh commandment: “You shall not commit adultery” (Exodus 20:14). The fact that the prohibition is simply stated with no explanation indicates that the meaning of adultery was well understood at the time Moses gave the law. Scripture is consistent in the prohibition against adultery.
In spite of the clarity of the original pattern of marriage and the prohibition against adultery, sinful humanity has developed ways to attempt to blur the lines of morality.
Polygamy is one way the prohibition against adultery has been to some extent circumvented. Polygamy is not technically adultery, although it does adulterate God’s original plan for marriage. In the Old Testament, polygamy was allowed by God but never endorsed by Him. Polygamy was not considered adultery because, although a third person (or perhaps a fourth, fifth, etc.) was added to the marriage, the additional women were legally included in the marriage. A polygamist who engaged in sexual activity with someone other than his legal wives was still committing adultery. Since polygamy is generally illegal in modern countries today, no third person can be legally added to a marriage.
Divorce and remarriage is another way that the prohibition against adultery has been circumvented. If a married man has an affair, he is committing adultery. However, if he divorces his wife and marries the other woman, then he maintains his “legal” footing. In most modern societies, this has become the norm.
Jesus puts both of these “strategies” to rest: “Anyone who divorces his wife and marries another woman commits adultery” (Luke 16:18). And, “Anyone who divorces his wife and marries another woman commits adultery against her. And if she divorces her husband and marries another man, she commits adultery” (Mark 10:11–12). According to Jesus, divorce does not circumvent the prohibition against adultery. If a married man sees another woman, desires her sexually, divorces his wife, and marries the other woman, he still commits adultery. Since the marriage bond is intended to last a lifetime, divorce does not release one from the responsibility to be faithful to the original spouse. (On a related note, we recognize that in some cases Scripture allows divorce, and, when divorce is allowed, remarriage is also allowed without being considered adulterous.)
Jesus carried the prohibition against adultery even further than the Mosaic Law: “You have heard that it was said, ‘You shall not commit adultery.’ But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart” (Matthew 5:27–28). So, even if a man tries to “legally” avoid adultery by seeking divorce, he is already guilty because of the lust in his heart that drove him to such measures. If a man “legally” brings another woman into the marriage, making it a polygamous marriage, he is still guilty of adultery because of the lust in his heart that motivated him to marry another wife. Even if a man or woman simply indulges in lustful thoughts (pornography is especially problematic), then he or she is committing adultery even if no extramarital physical contact ever takes place. This explanation by Jesus avoids all of the nuances about “how far is too far” with someone other than a spouse, and it avoids the need to define what “sex” really is. Lust, not sex, is the threshold of adultery.
Proverbs 6 gives some stern warnings against committing adultery, giving “correction and instruction . . . keeping you from your neighbor’s wife” (verses 23–24). Solomon says,
“Do not lust in your heart after her beauty
or let her captivate you with her eyes. . . .
Another man’s wife preys on your very life.
Can a man scoop fire into his lap
without his clothes being burned?
Can a man walk on hot coals
without his feet being scorched?
So is he who sleeps with another man’s wife;
no one who touches her will go unpunished” (verses 25–29).
Adultery is deadly serious and brings God’s consequences. “A man who commits adultery has no sense; whoever does so destroys himself” (Proverbs 6:32; cf. 1 Corinthians 6:18 and Hebrews 13:4).
A person who lives in unrepentant adultery gives evidence that he or she has not truly come to know Christ. But adultery is not unforgiveable, either. Any sin that a Christian commits can be forgiven when the Christian repents, and any sin committed by an unbeliever can be forgiven when that person comes to Christ in faith. “Do you not know that wrongdoers will not inherit the kingdom of God? Do not be deceived: Neither the sexually immoral nor idolaters nor adulterers . . . will inherit the kingdom of God. And that is what some of you were. But you were washed, you were sanctified, you were justified in the name of the Lord Jesus Christ and by the Spirit of our God” (1 Corinthians 6:9–11). Notice that in the Corinthian church there were former adulterers, but they had been washed clean from their sin, sanctified, and justified.