The seventh commandment is clear: “You shall not commit adultery.” The rabbis taught a narrow view of this commandment: the physical act of adultery. Jesus, the only perfect interpreter of the Law, went behind the physical act to the heart of the issue.

In Matthew 5:27-28, Jesus said, “You have heard that it was said, ‘You shall not commit adultery’; but I say to you that everyone who looks on a woman to lust for her has committed adultery with her already in his heart.”

These two verses constitute one paragraph of thought. Jesus is talking about a deep-seated desire that consumes a person mentally. It is not simply a look or a glance but an obsessive, continual looking that arises out of a heart of wrongful sexual desire. When an object of sexual desire appears (mentally or physically), this person’s already overactive and depraved imagination leads him into adultery in his heart.

Craig Blomberg has observed, “Adultery among Christians today is a scandal, yet it almost never occurs without precipitation” (without cause).

It is an advertising reality in our culture that “sex sells.” Immodesty, lewdness and sensual images are evident in nearly all types of media. Pornography is rampant, especially on the Internet. We are a sex-saturated society, much like ancient Corinth. Our culture embraces, desires and promotes permissiveness — and even perversion. Does a person lust because there is so much temptation, or do depraved hearts create the objects of sexual temptation in a society?

Jesus indicates that the problem is not external (though that is a problem) but internal — within our own hearts. This is also where the issue of lust must be resolved.

Lust is inordinate desire, or desire without restraint or discipline. There is a natural sexual desire in people that is normal, right and healthy. Marriage is the place where sexual relations are right. Everywhere else, it is sin. Adultery is serious. Sex outside of marriage is wrong. The rabbis condemned the physical practice of adultery, but Jesus says the desire that drives a man to imagine sex with a woman other than his wife is guilty of adultery also.

John MacArthur has written, “It is not lustful looking that causes the sin in the heart, but the sin in the heart that causes lustful looking. The lustful looking is but the expression of a heart that is already immoral and adulterous.”

David saw Bathsheba bathing on the roof. He could have looked away, but instead he continued to look at her, desire her and imagine being with her. He was a man of God and the king of Israel, but he was also a man with lust in his heart. He had Bathsheba brought to his chambers for sexual immorality. The act of sin was the culmination of the fruit of sinful desire that was already in his heart.

Human behavior has consequences, and David reaped the results of his lust.

Lust is an issue of the heart. What we think and how we handle our desires are vitally important. Adultery is damaging, and adultery in a person’s heart is destructive.

An old proverb still rings true today: Sow a thought, reap an act; sow an act, reap a habit; sow a habit, reap a character; sow a character, reap a destiny.

Adultery is serious. Remember: It starts in a person’s heart.