"Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness..." (Gal 5:19). The word adultery, according to Vines Expository Dictionary of Biblical Words is defined as one "who has unlawful intercourse with the spouse of another."
The Bible warns us of this insidious sin throughout its pages in both Old and New Testaments. Exodus 20:14, "Thou shalt not commit adultery." Note the severity of punishment of those who committed this sin under the Law of Moses, Leviticus 20:10, "And the man that committeth adultery with another man's wife, even he that committeth adultery with his neighbour's wife, the adulterer and the adulteress shall surely be put to death." Though this is not required of us under NT Law, we have to be impressed with the level of seriousness that God has placed in this matter.
We cannot overlook the use of the word adultery by the Lord in Jeremiah 3:8 speaking of the nation of Israel guilty of this sin. "And I saw, when for all the causes whereby backsliding Israel committed adultery I had put her away, and given her a bill of divorce." The concept of "unfaithfulness" is here explained. Like a jealous husband, our Lord, wanted his spiritual spouse to be faithful and repent of her spiritual adultery. The complete prohibition of idolatry is better understood when we realize that God has always wanted the undivided love of those who belong to Him (cf. Exodus 20:5; Deuteronomy 4:24). This helps us to better understand why adultery is wrong in our lives; because in the sin of adultery is this same faithlessness. If we recognize the need to be faithful to our physical spouse we will better understand the necessity of being faithful to our spiritual Husband (Ephesians 5:22-32).
Another dimension is added to this when we read Christ's words in Matthew 5:27-28, "Ye have heard that it was said by them of old time, Thou shalt not commit adultery: But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart." The way our Lord looks at this, the act of sexual intercourse does not actually have to physically have taken place, only contemplated in the thinking part of man. In such a case, which is common for many, adultery has already taken place and the condemnation of the sin with it. This should cause us to pay particular attention to the pastimes of our day like television, reading material, movies and the ever-present pornography offered on the Internet. So much evil has entered the formerly sacred domain of our homes in recent years as to make us all fear greatly and to resolve to avoid at all costs such evil influences that can cause us to commit this sin of the heart.
It is important for Christians to understand the nature of this particular sin and to know fully the consequences of committing adultery. 1 Corinthians 6:9-10, "Know ye not that the unrighteous shall not inherit the kingdom of God? Be not deceived: neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor abusers of themselves with mankind, Nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners, shall inherit the kingdom of God." If we want to have heaven, we must avoid such sin.
God made provision for us as our Creator to fulfill our sexual desires and needs. 1 Corinthians 7:2-4, "Nevertheless, to avoid fornication, let every man have his own wife, and let every woman have her own husband. Let the husband render unto the wife due benevolence: and likewise also the wife unto the husband. The wife hath not power of her own body, but the husband: and likewise also the husband hath not power of his own body, but the wife." If we utilize our spouse in the way God intended, and likewise for his or her benefit, the temptation to stray will be greatly lessened in both partners. Hebrews 13:4, "Marriage is honourable in all, and the bed undefiled: but whoremongers and adulterers God will judge."
In this regard let us reflect upon the words of the writer of godly wisdom found in Proverbs 5:15-20, "Drink water from your own cistern, and running water from your own well. Should your fountains be dispersed abroad, streams of water in the streets? Let them be only your own, and not for strangers with you. Let your fountain be blessed, and rejoice with the wife of your youth. As a loving deer and a graceful doe, let her breasts satisfy you at all times; and always be enraptured with her love. For why should you, my son, be enraptured by an immoral woman, and be embraced in the arms of a seductress?"