We need to guard our hearts above all else because our hearts can hold both good and evil. Jesus said that the righteous bring forth good out of the goodness stored up in their hearts, and the evil bring forth evil out of the evil in their hearts.
He had also once warned, "If your right eye causes you to sin, pluck it out and throw it away; it is better that you lose one of your members than that your whole body be thrown into hell. And if your right hand causes you to sin, cut it off and throw it away; it is better that you lose one of your members than that your whole body go into hell" (Mt 5:29-30).
Eyes and hands are essential parts of our body—it is difficult to survive without them. But Jesus' strong language teaches that if these organs cause us to go to hell, it would be better to get rid of them than to forsake our blessings in eternity.
The truth is, what causes us to sin has nothing to do with our eyes or hands, but rather the law of sin that rules in human hearts. Our eyes and hands are just the tools for sin. If we do not guard our hearts carefully above all else, there is no value in plucking out our eyes or cutting off our hands.
Satan knows that a superior method of spiritual warfare is to attack the heart. He knows that to ruin a person's spiritual life he must first rule that person's heart. Judas Iscariot, who perished in sin, did not guard his heart against greed: he often stole money from the disciples' treasury box. His greed eventually led him to betray Jesus; Satan entered his heart and told him to give Jesus over to the enemy for earthly gain.
The apostle Paul, on the other hand, knew all the conniving ways of Satan in his scheme to win the hearts of this world. Therefore, Paul devoted himself to overcoming his body and making it obedient to the will of God. In this way, Paul triumphed over all of Satan's schemes.
The heart is the source of our words and behavior, be they good or evil. It is the wellspring of a person's life. Therefore, if we can properly safeguard our heart, we will make fewer mistakes and accomplish more good.