The Torah warns us “not follow after your own heart and your own eyes, after which you played the harlot” (Numbers 15:39). Gazing on others with lust constitutes “following after your eyes.” Harboring immoral thoughts in the imagination constitutes “following after your own heart.” This is the adultery of the heart which Yeshua warned His disciples to avoid.
One might feel tempted to disregard Yeshua’s warnings as merely an expression of hyper-piety, but He emphasized the seriousness of the sin, warning of consequences in Gehenna. He said it would be better to cut off a member of one’s own body than to suffer the consequences after death.
An ancient explanation of the prohibition on adultery says that the Hebrew verb for “to commit adultery (tin’af, תנאף)” in Exodus 20:14 consists of four letters, in order to warn “lest you commit adultery with the hand, or with the foot, or with the eye, or with the heart.” Likewise Yeshua advised His disciples to cut off the offending hand or foot or pluck out the offending eye that leads them to commit adultery of the heart.
If your hand causes you to stumble, cut it off; it is better for you to enter life crippled, than, having your two hands, to go into hell, into the unquenchable fire, where their worm does not die, and the fire is not quenched. If your foot causes you to stumble, cut it off; it is better for you to enter life lame, than, having your two feet, to be cast into hell ... If your eye causes you to stumble, throw it out; it is better for you to enter the kingdom of God with one eye, than, having two eyes, to be cast into hell ... (Mark 9:43-48)
In no sense should the Master’s words be taken literally to imply that a man should maim himself. Rabbi Yeshua employed rabbinic hyperbole typical of the sages to express the severity of the sin. Adultery of the heart does spiritual damage to a man’s soul on the same caliber as if he had physically maimed himself. Paul says, “Flee immorality. Every other sin that a man commits is outside the body, but the immoral man sins against his own body” (1 Corinthians 6:18).