Forty years before the destruction of the Temple, Yeshua proclaimed a gospel of repentance to His generation: “Repent, for the kingdom of heaven is at hand.” He warned them that, if they did not repent, they would face a terrible judgment. He called upon the Jewish people of His day to turn from the sins of immorality and adultery.
The Master warned His disciples about a different type of adultery—an adultery that takes place in the mind. "Do not follow after your own heart and your own eyes" (Numbers 15:39). If a man gazes lustfully on another man’s wife, he has already committed adultery in his heart, that is to say, in his thoughts and intentions.
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 lust for her has already committed adultery with her in his heart. (Matthew 5:27-28)
The concern lies with the secret violation of another’s marriage. The Torah says, “You shall not commit adultery” (Exodus 20:14). The Torah defines adultery as sexual relations with a betrothed or married woman. On the basis of the Torah’s words, “a man shall be joined to his wife; and they shall become one flesh” (Genesis 2:24), Yeshua broadened the Torah's scope to include any breach of monogamous fidelity. He taught that even impure thoughts can constitute adultery of the heart.
James, the brother of the Master says, “When lust has conceived, it gives birth to sin” (James 1:15). The apostles transmitted the same principle to the early Gentile disciples: “My child, be not a lustful one; for lust leads the way to sexual immorality” (Didache 3:3).
Yeshua’s warning is consistent with similar warnings in Jewish literature. The Wisdom of Sirach says, “Veil your eyes before a beautiful woman, look not at another one’s beauty that does not belong to you” (Sirach 9:8). The second-century sage Reish Lakish seems to echo the words of Yeshua: “You must not suppose that only he who has committed the crime with his body is called an adulterer, if he commits adultery with his eyes he is also called an adulterer” (Leviticus Rabbah 23:12).
Sinful imagination leads to desire; desire to intent; intent to pursuit; pursuit to deed. This is to have you know how difficult it is for a person to turn back from one to the other. (Kalla Rabbati II, 6)
The punishment for committing adultery is death. Adultery of the heart is not the same as actual adultery. A Torah court of law on earth has no jurisdiction to try a man or sentence him for merely gazing on another man’s wife. Instead, Yeshua warned that adultery of the heart will be punished in the hereafter.