Jesus said to His disciples in Matthew 5:27-30,

“You have heard that it was said to those of old, ‘You shall not commit adultery.’  28 But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart.  29 If your right eye causes you to sin, pluck it out and cast it from you; for it is more profitable for you that one of your members perish, than for your whole body to be cast into hell.  30 And if your right hand causes you to sin, cut it off and cast it from you; for it is more profitable for you that one of your members perish, than for your whole body to be cast into hell.

In this passage from Jesus’ Sermon on the Mount, He indicates that the Pharisees and Sadducees had reduced the teaching of the seventh commandment, “Thou shalt not commit adultery,” to the mere physical act. They had forgotten the spirit of the law, and they also did not take that seventh commandment into the context of the other commandments. According to D. Martyn Lloyd-Jones, it should have been taken in conjunction with the tenth commandment, “Thou shalt not covet…” For this commandment shows us the true meaning of lust, and when you put the two together, they show the entire meaning of lust—to covet in lust and then also to carry it out in the act of adultery.

But let me be clear, the spirit of the law in this seventh commandment is much more than the physical act of adultery. It is essentially a lustful desire based in the heart, and then it sometimes expresses itself in a physical act. But that physical act is not even necessary to be sin. The lust in the heart is enough. The sad thing about this is that many fail to understand this. They deny their lustful heart and they are smug and satisfied with themselves because they have not committed the act. Hence, some really don’t think they have sinned.

As Christians we must always pay attention to our heart and mind. Our sins are not always evident by our actions. Thus, it is up to us to be watchful. Our holiness demands constant watchfulness and self-examination. And if we find in ourselves a lustful heart, take great effort and prayer to purify yourself.

Here are four reasons we need to be watchful of sin in the heart.

• Sin is like a disease and has great power to corrupt the whole being. Note that it is the disease that kills, not the symptoms. Hence if you want to get rid of your lustful actions, you must first examine you heart to find the root cause of the sin.
• Sin is subtle. Your sin will fool you into thinking that as long as you haven’t committed an act you are good. Sin in the heart is realized when we flush out why we want to commit it, or why we desire it. Also, ask yourself why you are reading certain material or why you desire to watch certain TV shows. Examine your motives.
• Sin perverts. Sins twists everything around. Our hands and our eyes may be given into its lust and may result in serious perversions (vv. 29-30).
• Sin is destructive. Sin may result in sickness of all kinds; and ultimately, for the unconverted, it may keep you from conversion and eternal life (v. 30).