In its truest sense, adultery is a sin against God that begins in the heart. Jesus said in Matthew 5:27-28, “You have heard that it was said, ‘You shall not commit adultery’; but I say to you that everyone who looks at a woman with lust for her has already committed adultery with her in his heart.”
Jesus also said in Mark 7:20-23, “That which proceeds out of the man, that is what defiles the man. For from within, out of the heart of men, proceed the evil thoughts, fornications, thefts, murders, adulteries, deeds of coveting and wickedness, as well as deceit, sensuality, envy, slander, pride and foolishness. All these evil things proceed from within and defile the man.”
However, practically speaking, adultery is committed when a married individual engages in sexual activity with someone other than their biblical spouse. Deuteronomy 22:22 says, “If a man is found lying with a married woman, then both of them shall die, the man who lay with the woman, and the woman.” Romans 7:3 states, “So then, if while her husband is living she is joined to another man, she shall be called an adulteress; but if her husband dies, she is free from the law, so that she is not an adulteress though she is joined to another man.” Additionally, Proverbs 6:32 says, “The one who commits adultery with a woman is lacking sense; he who would destroy himself does it.”
Infidelity is also committed when an individual remarries after divorcing their previous spouse for unbiblical reasons. Matthew 5:32 says, “But I say to you that whoever divorces his wife for any reason except sexual immorality causes her to commit adultery; and whoever marries a woman who is divorced commits adultery.” Hence, sexual immorality is one of only two allowances for a biblical divorce. (The other allowance for a biblical divorce is when an unbelieving spouse abandons the marriage. It states in 1 Corinthians 7:12-15, “But to the rest I, not the Lord, say: If any brother has a wife who does not believe, and she is willing to live with him, let him not divorce her. And a woman who has a husband who does not believe, if he is willing to live with her, let her not divorce him. For the unbelieving husband is sanctified by the wife, and the unbelieving wife is sanctified by the husband; otherwise your children would be unclean, but now they are holy. But if the unbeliever departs, let him depart; a brother or a sister is not under bondage in such cases. But God has called us to peace.”)
Other forms of sexual betrayal in a marriage that are not mentioned in Scripture (and may or may not "officially" qualify as adultery) are erotic phone calls with others, fantasizing about others during marital intimacy, visiting strip clubs, viewing explicit sexual content, engaging in extramarital foreplay, pursuing romantically-based extramarital relationships, or any other inappropriate behavior that violates the sacredness of marriage.
Ultimately, God’s desire for couples enduring the aftermath of adultery (or sexual betrayal) is for the unfaithful spouse to genuinely repent and consistently demonstrate their repentance and renewed faithfulness through their words and actions. This will help rebuild their spouse’s trust over time and facilitate their forgiveness and healing. Additionally, the violated spouse should work toward forgiving their partner so the marriage can eventually be restored with God’s grace and help.
However, if genuine repentance is absent or the damage is too severe for the violated spouse to overcome, God lovingly and graciously provided an allowance for divorce—but only as a last resort, not an automatic response or outcome.
I hope this information helps you understand what constitutes adultery from a biblical perspective and sheds light on other forms of sexual betrayal that can equally destroy a marriage.
