What is the 7th Commandment from the Bible and applications for today’s believer?
The Ten Commandments
The Ten Commandments were never part of the Old Testament or Mosaic Laws at all. These were written by the very finger of God and in stone, indicating their permanence, unlike the Mosaic Laws which were written on parchment or animal skins. These laws were only until the time of Christ and the completed work on Calvary. These laws all pointed forward to the coming work of Christ and were only shadows and shadows are not the reality. Jesus is the fulfillment of the law. The Mosaic Laws always were always directed at Israel as the Scriptures would say, “and thus say unto the children of Israel” or “to you Israel,” so these laws like unclean and clean foods, circumcision and others were for the nation of Israel because that’s who they were always addressed to, however the Ten Commandments were still being mentioned by Jesus and the other apostles in the New Testament  so the commandments of God are permanent and they are still relevant today and always will be, at least until the kingdom arrives.
The Seventh Commandment
When God gave the Ten Commandments to Moses, He also gave them to all people and not just to Israel because the Mosaic Laws would come later and be separate from the Ten Commandments. The 7th Commandment, as are all of the commandments, is given in Exodus 20 but repeated in several places in the Old Testament. Exodus 20:14 says “You shall not commit adultery” and the penalty of anyone breaking this commandment in Israel was death, but today, this commandment seems to be broken on a consistent basis, tragically, even among professing Christians. How seriously did God take this law? Leviticus 20:10 says “If a man commits adultery with the wife of his neighbor, both the adulterer and the adulteress shall surely be put to death.” God has not changed and neither has His law about adultery, so even though it seems many today are “getting away with it,” the day will come when this great sin is brought before them as “each of us will give an account of himself to God” (Rom 14:12). Their judgment will come at Christ’s second appearance or after they die, when they’re judged (Heb 9:27), whichever comes first. Committing adultery is deadly serious to God and these unrepentant sinners will join “the cowardly, the faithless, the detestable, as for murderers, the sexually immoral, sorcerers, idolaters, and all liars, their portion will be in the lake that burns with fire and sulfur, which is the second death” (Rev 21:8).
Keeping the Commandments
Jesus actually amped up the requirements of the Ten Commandments and to say that Jesus did away with the commandments is utter nonsense because he ran the rich man through the commandments because this man thought he was “good person” when none are at all (Rom 3:10-12). A rich man came up to Jesus and asked “what must I do to inherit eternal life” (Mark 10:17b) but he didn’t understand that it’s not what we do to be saved but how we can be saved which is by grace alone (Eph 2:28-9), so Jesus says to him, “You know the commandments: ‘Do not murder, Do not commit adultery, Do not steal, Do not bear false witness, Do not defraud, Honor your father and mother” (Mark 10:19) and the rich man boasted, “Teacher, all these I have kept from my youth.” And Jesus, looking at him, loved him, and said to him, “You lack one thing: go, sell all that you have and give to the poor, and you will have treasure in heaven; and come, follow me.” Disheartened by the saying, he went away sorrowful, for he had great possessions” (Mark 10:20-22). Jesus’ heart for this young man hurt because He loved him and knew that he was making his money his own God, so the rich man hadn’t been keeping all of the commandments at all because his money was his god. By the way, every other religion in the world is “do, do, do” but in Christianity , it’s “done!” No amount of commandment keeping could ever save us but this doesn’t mean that we are free to break them, for if we break them, they will break us.
Committing Adultery in the Heart
Jesus must have stunned His audience when He was speaking about adultery because He said, “You have heard that it was said, ‘You shall not commit adultery.’ But I say to you that everyone who looks at a woman with lustful intent has already committed adultery with her in his heart” (Matt 5:27-28). Jesus said that it’s not just the physical act of adultery but the lusting in the heart to the point of fantasizing about it with someone else’s spouse. In other words, sin begins in the heart as it is “out of the heart come evil thoughts, murder, adultery, sexual immorality, theft, false witness, slander” (Matt 15:19). If someone lusts after another, even in their heart, God knows about it and that’s seen as adultery as far as God is concerned. Yes, the physical act is worse but the mind conceives of sin and then later gives birth to sin and sin brings death (Rom 6:23) and we can sin inwardly without ever doing it outwardly.
Joined with a Prostitute?
When God joined Adam and Eve in marriage, the two became as one in the flesh (Gen 2:24) and the same thing happens when a man or woman has sex with someone who is not their spouse. The Apostle Paul asked, “Do you not know that your bodies are members of Christ? Shall I then take the members of Christ and make them members of a prostitute? Never! Or do you not know that he who is joined to a prostitute becomes one body with her? For, as it is written, “The two will become one flesh” (1 Cor 6:15-16) and the fact is “God will destroy both one and the other. The body is not meant for sexual immorality, but for the Lord, and the Lord for the body” (1 Cor 6:13).
When you are tempted by seeing another man or woman who is not your spouse, then do what Job did; “I have made a covenant with my eyes; how then could I gaze at a virgin” (Job 31:1)? He made a vow that when he looked upon a young woman, he would look away. That’s still great advice for today. If not, Paul’s speaks to you about “the works of the flesh are evident: sexual immorality, impurity, sensuality, idolatry, sorcery, enmity, strife, jealousy, fits of anger, rivalries, dissensions, divisions, envy, drunkenness, orgies, and things like these. I warn you, as I warned you before, that those who do such things will not inherit the kingdom of God” (Gal 5:19-21).
Consider reading this article too: What is the Biblical Definition of Adultery? 
Resource – Scripture quotations are from The Holy Bible, English Standard Version® (ESV®), copyright © 2001 by Crossway, a publishing ministry of Good News Publishers. Used by permission. All rights reserved.