“But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart” (Matthew 5:28).
As Jesus comments on the implications of the law for kingdom living, He turns from murder to adultery. Many of the Pharisees acted as if only the physical act of adultery was forbidden by the Ten Commandments, and that “everything but” physical adultery was permitted. Jesus pointed to lust in the heart as also forbidden.
I’m sometimes asked by college students how “far” it is all right to go in sexual intimacy ("Oral is moral, right Mr. Dowling?"). Jesus answers that question right here. It is not permitted to go any distance at all outside of marriage, because all forms of sexual intimacy, and even lust, are condemned outside of marriage.
Some have taken Jesus’ admonitions the wrong way, and have thought that Jesus was saying that lust in the heart is just as bad as fornication and adultery. This is not the case at all. In the Old Testament law, given by the same God who sent Jesus, adultery could be punished by the death penalty (Deuteronomy 22:22). Obviously, lust in the heart was not punished that way.
Similarly, Paul admonished the Corinthian church because it failed to excommunicate a man who had committed adultery (1 Corinthians 5). Church discipline is not to be measured out against those who go no farther than to lust in their hearts. In the same way, in the Bible lust is not grounds for divorce, but adultery is.
Roman Catholic theology distinguishes between mortal and venial sins. A mortal sin removes your justification, and if you die with unconfessed mortal sin on your soul, you will be sent to hell. Venial sins do not destroy your justification, and only reduce your rewards or add to your time in purgatory. The Reformation rejected this system because of its works-orientation but did not reject the idea of degrees of sin. John Calvin said that all sin is mortal in the sense that it deserves death, but no sin is so severe that it can destroy the grace of justification. All sins are serious, though some sins are worse than others.
Before God all sins are serious. Today take a good look at your sexual life. If you are not married, are you keeping yourself pure in the sense that Jesus requires? If married, are you giving yourself to your spouse wholeheartedly? In all cases, keep your thoughts honoring to Christ.
