Thou shalt not commit adultery. Like a set of Russian dolls, the commandments on the second table of the law have presuppositions that are nested within each other. Israel’s relationship with God had implications for the national community, and of course nations are made up of individuals and their various relationships.
The fifth commandment presupposes the parent/child relationship; the sixth, eight, and ninth assume that your neighbor’s life, personal property, and character are not yours to take. The seventh command assumes a certain kind of relationship: a marriage.
The sort of marital relationship which this commandment presupposes is a sexual covenant between a man and a woman, which is to be observed faithfully. The marriage vows and sexual consummation bind a man and women together as one flesh, until the covenant is dissolved by death. Adultery is, very simply, unfaithfulness to the marriage vow.
If you are not covenanted with someone in marriage, you have no lawful justification for a sexual union or relationship with them. That simple. Notice, how clear and unambiguous the commandment is, and how polluted and deliberately ambiguous the human heart is in regards to sexual purity. Does internet porn count? Lingerie brochures? Mental fantasies? Long phone calls, or text conversations? Is marriage really just one man and one woman? If a hotter babe or hunkier guy comes along can I dump my spouse to enjoy a fling? What if my spouse is ok with an “open marriage?” Wouldn’t God want me to be happy with my soul-mate, rather than stuck in an unhappy marriage with this person I’ve come to detest?
The OT prophets often accused the nation of Israel of being adulterous because of their compromise with idols. They asserted unfaithfulness to God went deeper than mere external conformity to His outward commands; their unfaithfulness started in the heart. In Jesus’ teaching on adultery in the Sermon on the Mount, he goes further, and attacks even the seeds of adultery: lust, desire, and––behind all of that––discontentment.
Discontentment drives us to adulterate our marital and spiritual duties.Discontentment drives us to adulterate our marital and spiritual duties. It seeks the illusion of earthly satisfaction at the well of sexual gratification.Spiritual adultery seeks the illusion of union with God at the altar of idols. However, God wants us to find eternal joy in Him. We see the shadow of the goodness of His covenantal faithfulness in earthly marital fidelity. Remain faithful in your marriage, yes. But above all, let marital faithfulness be derivative of spiritual faithfulness: worshipping the Father, through His Son, Jesus Christ, by the power of His Spirit.