Any sexual act between a married woman and a man who is not her husband, or between a married man and a woman who is not his wife. This breaks the unity of marriage.
In Old Testament times, having multiple spouses was not considered adultery (Deuteronomy 21:15). It also was not adultery if a husband had sexual intercourse with his female slave (Genesis 16:1–4; 30:1–5).
Jesus rejected these imbalances between men and women in his teaching on divorce and remarriage. He allowed for divorce in cases of adultery (Matthew 5:32; 19:9). However, Jesus warned that in all other cases, remarriage after divorce was adultery. Paul added that this applies only if the original partner is still alive (Romans 7:2–3).
Jesus expanded the Old Testament’s definition of adultery by including people’s thoughts. Any man who thinks lustfully (beyond being tempted) has committed adultery in his mind, even with no physical contact (Matthew 5:27–28; compare Job 31:1, 9).
The Bible strongly condemns adultery throughout the Old Testament law, prophecy, and wisdom literature.
The Ten Commandments forbid adultery (Exodus 20:14; Deuteronomy 5:18).
The prophets say adultery angers God (Jeremiah 23:11–14; Ezekiel 22:11; Malachi 3:5).
Proverbs describe adultery as self-destructive (Proverbs 6:23–35; compare 7:6–27).
The New Testament continues this condemnation. Without repentance, adultery excludes people from God’s kingdom (1 Corinthians 6:9). Adultery is the opposite of loving one’s neighbor (Romans 13:9–10). God will judge adulterers (Hebrews 13:4).
In the Old Testament, the penalty for adultery is death for both the man and the woman (Leviticus 20:10; Deuteronomy 22:22). This is the case whether a woman engaged to another man, other than in the cases of rape (where only the man is to be killed—Deuteronomy 22:23–27). The command “You must purge the evil from among you” (Deuteronomy 22:24) shows that adultery was a threat to society’s health, as well as a threat to the families of the two guilty parties.
Because the consequences were so serious, guilt had to be certain. When adultery was only suspected, the wife had to be tested by taking an oath and drinking bitter water. Since she stood in the Lord’s presence, the result was believed to reveal the truth (Numbers 5:11–31).
In both the Old and New Testaments, adultery is used symbolically to describe human unfaithfulness to God. The Old Testament prophets connected God’s covenant relationship with his people to marriage (Isaiah 54:5–8; compare Revelation 21:2). Breaking that relationship, especially by worshiping idols, was spiritual adultery (Jeremiah 5:7–8; 13:22–27; Ezekiel 23:37).
Jesus also used this idea of spiritual adultery to describe the people who rejected his claims or demanded proof of his divine nature (Matthew 12:39; 16:4; Mark 8:38). In James 4:4, God is described as a loving, jealous husband who deals with his adulterous people who are friends with the world.
The prophet Hosea focuses on this theme in a special way. God used Hosea's personal experience to teach an important lesson. Hosea's wife was unfaithful to him, just as God's people were unfaithful to God. This story shows:
How serious it is when God's people are unfaithful to him (Hosea 2:2–6)
How much God wants to restore the relationship (3:1–5)
Being unfaithful to God (spiritual adultery) leads to God's judgment, just like physical adultery. However, in both cases, God's strongest desire is to repair the relationship. This happens when people truly turn away from their wrong actions and come back to God (Jeremiah 3:1–14; Ezekiel 16:1–63).