The central, devastating truth is this: acts of sin spring not from external causes but from internal rot. As Jesus says in Matthew 15:19, “For out of the heart come evil thoughts, murder, adultery, sexual immorality, theft, false witness, slander.”
Our problem is much, much worse than we imagine. Sin isn’t simply what we do; it’s what we are at the core of our natural being. Every wicked thought, word, and deed flows from a foul fountain of evil within us. We are not sinners because we sin; we sin because we are sinners. It is our nature. Jesus’ indictment is dreadfully simple: the sin we commit oozes and erupts from an already sinful heart.
In biblical terms, the “heart” includes not only our emotions and desires but our thinking and choosing as well. The heart stands for everything that makes up the internal motor that drives what we do and say. I recently heard someone describe the workings of the inner man like this: “What the heart desires, the will chooses, and the mind justifies.” That’s true—but biblically speaking, these are all actions of the heart. The affections of the heart direct the choices of the heart, and the mind of the heart fabricates “reasons.” And all of it is bent and warped by sin.
Dr. A. Craig Troxel helpfully talks about the brain as hardware and the heart as software. The hardware is clearly engaged in our speaking and doing (though it doesn’t always appear to be). But it’s all driven by the software, by the heart. And that is where the problem lies. Every naturally conceived man and woman in human history is infected with this guilt-inducing, death-bearing heart virus (Rom. 3:9–20). We can neither fully understand it (Jer. 17:9) nor fix it (Jer. 13:23). Both the sin within and every single vile word and act it spawns are “liable to the hell of fire” (Matt. 5:22).
The severity of the human sin problem is impossible to overstate. “If then the light in you is darkness, how great is the darkness!” (Matt. 6:23). If the source of every external transgression is an inner irreparable evil condition, then how can we be saved? How shall we escape the wrath that is to come?
The beauty of a correct understanding of sin is that it destroys every hope of self-salvation. I simply can’t fix or atone for the sin that dwells in me. My one and only hope is the atoning blood, imputed merit, and transforming power of Jesus.