Adultery

Adultery (Heb: נָאַף, na'aph) is a act or state of sexual unfaithfulness to one's husband or wife. It is a sin against the sacred bond between a man and his wife. It is a major sin, first forbidden outright as the seventh commandment as written on stone by Yahweh. Under the Levitical code, those convicted of this crime were subject to the death penalty

The related sin of fornication (זָנָה zanah, "to commit whoredom"), often with hired lovers, led to Yahweh calling acts of idolatry spiritual adultery.

Laws
The first and clearest prohibition of adultery by God is the seventh commandment which simply states, "You will not commit adultery".

Death Penalty
Leviticus 20:10 states that if a man commits adultery with another man’s wife, "even he who commits adultery with his neighbor’s wife," both the adulterer and the adulteress are to be put to death.

David and Bathsheba
In 2nd Samuel chapter 11, King David had an affair with Bathsheba, the wife of Uriah the Hittite, while Uriah was off to war during the siege of the Ammonite city of Rabbah. The affair was complicated in that Bathsheba became pregnant with David's child, so David had Uriah taken off the battlefield in the hopes that he would go home and sleep with his wife. However, Uriah was too loyal to the king and refused to enjoy the privilege of doing so while his fellow soldiers were still fighting. David tried to get Uriah drunk enough so that he would get him to go home and sleep with his wife, but Uriah still refused to do so. As a last-ditch effort to keep the affair secret, David had Uriah sent back into battle with a message to his commanding officer Joab to be put in the forefront of the battle where the fighting was fiercest, in the hopes that Uriah would die in combat. Joab sent a few of his men with Uriah to the forefront, where they were all killed, and reported the message back to King David, who dismissed the matter as trivial. After a period of mourning, Bathsheba became David's wife and gave birth to a son.

However, the thing that David had done displeased the Lord, so He sent Nathan the prophet to tell a story about a rich man who took a poor man's sole prized possession, a lamb, and had it prepared for a guest's dinner. Enraged at hearing this, David said that the man who did this deserved to die and must pay back fourfold of what he had taken from the poor man. Nathan revealed that the story was about David's affair with Bathsheba and his subsequent arranged murder of her husband, and that God knew about it. For this, David was told that "the sword would never leave his house" and that his wives would be given to "a neighbor" who would sleep with them "before the sun". David confessed that he had sinned against the Lord. Nathan told David that the Lord had forgiven his sin, but David's infant son with Bathsheba was fated to die because of David's actions. Soon enough, David's son became sick, and David fasted, hoping that God would be gracious and change His mind about the child. But after several days when he found out that the child was dead, David accepted the Lord's decision was final, and after a period of mourning, David had another child with Bathsheba, which became Solomon, who would later succeed him as king.

Hosea and Gomer
Hosea the prophet was commanded by God to marry "a woman of adultery", so he married Gomer the daughter of Diblaim, who had three children during the marriage, although it is questionable whether two of those children were fathered by Hosea. God used this marriage along with Gomer's adulterous behavior as a picture of Himself dealing with Israel's constant unfaithfulness to Him as they committed spiritual adultery with idols.

The Woman Caught in Adultery
In John 8:1-11, Jesus dealt with the Pharisees bringing forth a woman that they claimed was caught in the act of adultery. However, they did not bring forth the man with whom she supposed to have committed adultery. Nevertheless, they hoped to use this case to have Jesus say something of whether the woman should be stoned or not as a basis for an accusation. Jesus purposely delayed in His answer before He told them, "Whoever is without sin among you, let him first cast a stone at her." This answer made the ones who brought up the charge depart from the scene, having the conscience convicted, until only Jesus and the woman were left. Jesus asked the woman, "Where are your accusers? Has no one condemned you?" The woman answered, "No one, Lord." Then He said, "Then neither do I condemn you. Go and sin no more."

Lust
In the Sermon on the Mount Jesus extended the application of the prohibition against adultery (in Ten Commandments) to apply to lust. Considering the basis behind the prohibition of adultery, Jesus states how gazing upon a woman with the intent of sexually desiring her constitutes adultery (that is, not mere attraction but intentionally subjecting sexual thoughts onto another person). In Jesus view, having a lustful desire after a woman is just as bad as actually having adultery with the woman (could be applied to a man).

In order to highlight the grave danger of "adultery in the heart", Jesus uses the hyperboles of cutting of the right arm or right eye to prevent from lust leading to the ultimate judgement of Gehenna. If, something in a person's life entices them to sin (in this context, sexual lust) the person should take extreme measures to remove it and thus save themselves from Hell.

In Proverbs
Adultery, specifically the adulterous woman is a major theme in Proverbs. Near the beginning of the book, several lengthy expositions are provided concerning adultery as well as many maxims throughout the collection.

Second Exposition
Continuing the second exposition against committing adultery, the author of the Proverb (presumably Solomon) offers a story to warn against adultery- whether real or imagined. As the narrator peered through the lattice of his home, he noticed a young man on the street corner where an adulteress lurks. She meets the man wearing seductive clothing and kisses him. Enticing him through the promises of pleasure and avoiding the wrath of her traveling, merchant-husband, she convinces him to come into her bed chambers.