Before answering this question, it is important to clarify a difference between the Old Testament and New Testament. Under the Old Covenant law, given to ancient Israel under a theocracy, the punishment for adultery was death (Leviticus 20:10). In the New Testament, Jesus brought a new law into effect. The wages of sin is still eternal death (Romans 6:23), but adultery no longer carries the death penalty civilly. Modern Christians are not living under the old theocracy and are not commanded to harm those who sin.
The Old Testament law lists a number of behaviors that were punishable by death, including adultery. “If a man commits adultery with another man’s wife—with the wife of his neighbor—both the adulterer and the adulteress are to be put to death” (Leviticus 20:10). It is important to note that the punishment was the same for both parties involved. There was no double standard that made allowances for a man’s dalliances; he was punished right along with the woman. This law and others concerning sexual immorality in Leviticus 20 are tied to the need for the complete moral separation of Israel from other nations. The Canaanites had been known for their sexual licentiousness, among other things, and God wanted Israel to be holy, or “set apart,” from them (verses 22–24). Again, this law was given to Israel as part of the Mosaic Covenant. The church is not Israel, and we are not living under the Old Covenant.
Today, the Bible does not recommend any such punishment for adultery. However, the act of adultery carries its own punishment. Sexual sin is an offense committed against one’s own body (1 Corinthians 6:18). The book of Proverbs warns of the consequences of adultery: loss of honor and strength (Proverbs 5:9–11), a ruined reputation (Proverbs 5:14), bondage and death (Proverbs 5:22–23), self-destruction (Proverbs 6:32), and the vengeance of a jealous husband (Proverbs 6:34). “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” (Proverbs 6:27–29).
The Proverbs also outline the character of the adulterer: he is called simple and senseless (Proverbs 7:7) and compared to an animal caught in a snare and then slaughtered (Proverbs 7:22–23). “A man who commits adultery has no sense; / whoever does so destroys himself” (Proverbs 6:32). Finally, the proverb writer comes to this terribly grim conclusion about adultery: “Many are the victims she has brought down; / her slain are a mighty throng. / Her house is a highway to the grave, / leading down to the chambers of death” (Proverbs 7:26–27).
A reading of these warnings in Proverbs should be enough to strike fear into anyone’s heart. As drastic as the Old Testament law seems regarding the punishment for adultery, the spiritual consequences are even worse. Thankfully, the sin of adultery is not exempt from Jesus’ promise of forgiveness. We have only to look to the story in John 8 about Jesus’ interaction with an adulteress—caught in the very act and dragged before Him by the Pharisees—to see God’s heart toward the one trapped in the snare of sin. The Pharisees are ready and eager to exact merciless punishment upon the woman (but not the man), and Jesus rebukes them by reminding them that they are just as sinful as she. Then, when they have all walked away from the scene, He gently asks her, “Woman, where are they? Has no one condemned you?” and she says, “No one, sir,” to which He answers, “Then neither do I condemn you. . . . Go now and leave your life of sin” (John 8:10–11).
Jesus is full of grace and truth (John 1:14). He tells the woman to stop committing adultery, and He forgives her. This is a wonderful picture of John 3:17: “God did not send his Son into the world to condemn the world, but to save the world through him.” The punishment for adultery, or for any other sin, is wiped away when we receive Christ’s taking of that punishment for us.