“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. … “It was also said, ‘Whoever divorces his wife, let him give her a certificate of divorce.’ But I say to you that everyone who divorces his wife, except on the ground of sexual immorality, makes her commit adultery, and whoever marries a divorced woman commits adultery.”
Matthew 5:27-28, 31-32
Never forsake your commitment to your spouse. Do not even think about others lustfully in your heart. When our eyes stray from the things of God to the things of this world, we can lose our perspective and make harmful choices.
The most intimate of human relationships contains an indication of our relationship with our Creator. The act of adultery is an affront to the holiness of God, as it is a violation of one of the base commandments. Yeshua carries this base commandment even further into the realm of its origin, in our thoughts. The wrong thoughts lead to wrong actions, and wrong actions are sin.
In the Bible, adultery, while wrong in and of itself, is a metaphor for idolatry. Time and again, Israel’s unfaithfulness with the gods of the surrounding nations is compared to adultery with God.
We are urged to maintain our holiness, being set apart for the purpose of God, by keeping ourselves from being swept away by the lure of the created things that would distract us from our true purpose. Keeping our thoughts pure keeps us from sin.
The Core of the Bible 