You are probably familiar with the story of David and Bathsheba. David, the king of Israel, stays home from battle (when he should have been leading the charge). During this lapse in leadership, he sees Bathsheba, desires her, impregnates her, and then tries to cover it up, eventually resulting in the murder of her husband (See 2 Samuel 11 for more details). Later he is confronted by the prophet Nathan and he confesses and repents of his sin.
Psalm 51 is the song that David writes after this confrontation and throughout the psalm David writes about the condition of his heart. He asks God to create a new heart and renew a right spirit within him (v. 10). He pleads with God to give him a “willing spirit” (v. 11) and acknowledges that God delights not in sacrifices but in “a broken spirit, a broken and contrite heart” (v. 12). David recognizes that although the sin action was physical, it didn’t start with the physical act. It came from a heart that was focused on what he desired rather than what God did.
The same is true for us. When we sin, it demonstrates a heart condition that existed prior to the sinful act in which we engaged. The visible evidence of our sinful nature only confirms that the condition existed; it doesn’t create it. When our hearts are not right before God, when we’ve replaced Him as the priority, that’s when we act in ways that are contrary to His Word. That’s when we act in ways that are contrary to Him.
That’s why it is important that we guard our hearts. We do so not only to experience the sweetness of fellowship with our Savior, but because in doing so we are putting up barriers against future sin. We protect what our heart ingests and what it consumes because while we may think that it is not effecting us, experience and Scripture say that it does. Maybe not immediately, and maybe not in obvious ways, but if sin starts with the heart, then what we let into our heart are either the ingredients for obedience, or for sin.
This requires vigilance. As David learned, one poor decision can lead to others, until eventually he was ensnared in a conspiracy of massive proportions. The small choices we make – to give into a lazy disposition, to laugh at an inappropriate joke, to obscure the truth – have consequences for our future actions as well. But the impetus for those choices is found in the degree to which we protect our hearts, because ultimately that’s where sin, and obedience, both start.