Before we can answer this question, we need to be clear on the definition of  adultery. The dictionary defines “adultery” as “voluntary sexual intercourse  between a married person and a person who is not his or her spouse.” The Bible  would concur with this definition. In Leviticus  18:20, God told Moses, “Do not have sexual relations with your neighbor’s  wife and defile yourself with her,” and in Deuteronomy 22:22, we find a similar definition: “If a  man is found sleeping with another man’s wife, both the man who slept with her  and the woman must die.” It is clear from these definitions that adultery refers  to a consensual sexual union. What is not explicitly clear is whether or not  both partners in this illicit sexual union are married. The biblical commands  prohibit a man from taking another man’s wife, but do not indicate whether or  not the man is also married. Be that as it may, it is safe to say that if a  person who is married willingly seeks a sexual encounter with another person,  whether or not that person is also married, both people are guilty of committing  adultery.

God’s reasons for instituting His commandment against adultery  are two-fold. First, God established the institution of marriage as being  between one man and one woman (Genesis  2:24; reiterated by Jesus in Matthew  19:5 and parallel passages). God created marriage to be the building block  of His creation and of society. Even after the fall (Genesis 3), marriage is  still a sacred union and the foundation for society. In marriage, the full  expression of the image of God is made manifest as the man and the woman  complement and complete each other. The Bible also teaches us that marriage is  the vehicle through which God designed the procreation of the human race and the  preservation of godly offspring (Genesis  1:28, 9:1; Malachi 2:15). With such a  premium placed on marriage, it’s no wonder God would seek to protect this union  from defilement (Hebrews  13:4), and thus prohibit adultery, which is the violation of the sacred  marriage union.

The second reason for the commandment is found in Leviticus  18:1-5. As God’s chosen people, the Israelites were to reflect God’s  character in the Promised Land. God commanded His people to be holy for He is  holy (Leviticus  11:44), and part of holy living is sexual purity. God did not want His  people emulating the behavior of the Egyptians from whom He delivered them, nor  did God want His people copying the behavior of the people into whose land He  was bringing them. The implication was that adultery (and other sexual sins) was  commonplace in the lands where the Israelites had been and were going  to.

So now we know what adultery is and why God instituted this command.  Finally, we need to learn what God meant by the command itself. As with all of  the Ten Commandments, there are things we need to avoid doing (the negative part  of the command) and things we need to be doing (the positive part of the  command). The negative part of the command is self-explanatory: Do not commit  adultery. However, there is more to this command than the simple avoidance of  extramarital relationships. One can make the argument that wrapped up in this  prohibition are all sorts of sexual sin (e.g., incest, fornication,  homosexuality, etc.), and that argument can be made on the basis of chapters  such as Leviticus 18. Also important is avoiding things that would lead or tempt  one to consider adultery, such as the unnecessary withholding of conjugal rights  (1  Corinthians 7:1-5). Jesus, in His Sermon on the Mount, made further  clarification of this command (Matthew  5:27-30) by including all kinds of lustful thoughts. Fantasizing about  having sexual relations with someone is the same, in God’s eyes, as actually  committing adultery. Therefore, we must avoid all things that would create  within us lustful thoughts (e.g., suggestive songs, sensuous movies,  pornography, etc.). We should also avoid immodest clothing or anything that  might cause a brother or sister in the Lord to stumble in this area (1 Timothy 2:9; 1 Peter 3:3).

The  positive part of the command would entail doing the opposite of what the command  prohibits: chastity in body, mind, words and action; keeping watch over what we  take in with our eyes and the other senses; an attitude of temperance and  self-control (i.e., moderation); being discerning over the company we keep;  dressing modestly; and fulfilling our marriage vows in regards to sexual  relations and cohabitation. Regarding sexual sin, the Apostle Paul said, “Flee  from sexual immorality. Every other sin a person commits is outside the body,  but the sexually immoral person sins against his own body” (1 Corinthians 6:18).  When it comes to sexual sin, the best course of action is to remove ourselves  from temptation and avoid such situations altogether.

Adultery is the  complete corruption of God’s good creation of marriage. Through the sin of  adultery, Satan tempts us to seek sexual fulfillment in avenues other than the  one God has ordained—within the bounds of monogamous, heterosexual marriage.  Adultery rips at the fabric of society because it tears apart marriages and  families which are the building blocks of society. God’s law in general, and the  7th commandment in particular, is held up as the standard for Christian  behavior.