The best place to begin is at the beginning; we'll start with Adam and Eve. God created Adam first, let him hang around by himself with no other humans long enough to figure out he was lonely, and then He created Eve. I have the feeling when God presented Eve to Adam that He was rubbing His hands with glee, in great anticipation of Adam's reaction to this exquisite, beautiful creature, created just for him. I love that the NIV reads that God brought her to Adam. "Ta-da! Look what I've made for you!"God apparently meant for them to be committed, lifelong companions. Some period of time down the road He didn't go back to Adam and declare, "Here, Adam, I made another woman for you. I know you were tired of Eve and she didn't meet all your wants and needs, so here, try this new woman." To quote a well-used phrase in one of our children's books, "That's NOT what the Bible says!!" (Nor did he offer another man to Eve, for that matter.)
When God took the time to write the Ten Commandments with His own hand, twice, one gets the idea that He was serious about those commandments. The seventh one, "You shall not commit adultery", is pretty straightforward.
Twice in the NIV the phrase "the wife of your youth" is used. The first incidence is used in Proverbs 5, delightfully so. This proverb is rightfully entitled "Warning against adultery", as verses 15-18 illustrate:
"Drink water from your own cistern,
flowing water from your own well.
Should your springs be scattered abroad,
streams of water in the streets?
Let them be for yourself alone,
and not for strangers with you.
Let your fountain be blessed,
and rejoice in the wife of your youth."
The writer exhorts husbands to be faithful to their wives, and continue to rejoice in and be content with the wife they were given in their youth. The implication is that the couple had been together for some amount of years, long enough to have aged together.
The other occurrence of this phrase occurs in the NIV have no poetry in them, just straight exhortation. Malachi addresses some who were wondering why God was ignoring their tearful pleas and requests:
"You ask, “Why?” It is because the LORD is acting as the witness between you and the wife of your youth, because you have broken faith with her, though she is your partner, the wife of your marriage covenant. Has not the LORD made them one? In flesh and spirit they are his...So guard yourself in your spirit, and do not break faith with the wife of your youth." (Malachi 2:13-15) God is witness to our marriage vows and expects us to be faithful to our spouses, as He is always faithful to us. He is faithful to us even when we are unfaithful to Him.
Perhaps the most serious verses against adultery are in I Corinthians 5. Paul was appalled that a man in the congregation was involved in adultery--with his father's wife no less. Not only does he chastise the Corinthians for not putting the offender out of fellowship, but he also exhorts them to deal with this offense in a very serious manner. He charged them: "When you are assembled in the name of our Lord Jesus and I am with you in spirit, and the power of our Lord Jesus is present, hand this man over to Satan, so that the sinful nature may be destroyed and his spirit saved on the day of the Lord."
Adultery is a grave offense. Apparently this sin, and the prideful attitude of the offender, was so serious that the Corinthian church deliberately involved the unseen spiritual world in the destruction of his sinful nature for the salvation of the offender's spirit. The offender had to go through a purging and refining fire for the adultery.
Paul goes on to say that Christians should not even associate with someone who calls himself a brother but is sexually immoral, and "With such a man do not even eat." This is sobering! Paul wrote a similar thing in II Thessalonians 3:14-15, "Do not associate with him, in order that he may feel ashamed. Yet do not regard him as an enemy, but warn him as a brother."
The good news is that if adultery is repented of, like other sins, it is forgivable. In John chapter 8, a woman was brought before Jesus by the Pharisees with the charge that she had committed adultery. They seemed eager to stone her for the offense.
However, Jesus, seeing her shame, had mercy on her. Instead of picking up a rock to throw at her, he told the crowd that anyone who was sinless could throw the first stone. They all admitted to themselves that none of them were without sin and left. Jesus then told her she was free to go, and charged her, "Go now and leave your life of sin."
Even though adultery severely betrays the marriage relationship, God is able to bring restoration and redemption of a damaged marriage and also to the offender. God takes adultery very seriously. If you're currently engaging in this sin, remember His faithfulness to you, and repent. Ask Him to enable you to extricate yourself out of this wrong relationship. "The one who calls you is faithful and he will do it," I Thessalonians 5:24.
Although adultery is a serious offense, if you've repented from it, you are forgiven! God is faithful when we ask Him to forgive. "If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness," I John 1:9. He puts our sins behind His back and remembers them no more. If God, very God, can do that, then we can accept and live in His forgiveness. Then we can claim the promise, "There is therefore now no condemnation for those who are in Christ Jesus", Romans 8:1.