The word “sin” first appears in the Bible when God speaks to Cain, warning him not to give in to his anger against his brother: “If you do not do well, sin is crouching at the door. Its desire is for you, but you must rule over it” (Genesis 4:7). This is not the first sin, though. Romans 5:12-14 teaches that Adam’s fall was also a sin and, of course, the origin of all human sin.
But what precisely is sin? The Westminster Shorter Catechism says it well: “Sin is any want of conformity unto, or transgression of, the law of God.” In Genesis 3 this involves Adam and Eve eating the fruit of the tree of the knowledge of good and evil despite God’s clear prohibition against doing so. In chapter 4 it takes the form of murder when Cain kills Abel. Through the rest of the Bible we see countless other expressions of sin: lying, stealing, idolatry, and impatience and anything else the heart of man can concoct. But what is the root that unites these behaviors? What is it about these actions that makes them sinful?
Any time we see sin, whether in Scripture or in our lives today, the heart of it is willful contempt for God. As John Piper writes,
The exceeding evil of sin is not the harm it does to us or to others (though that is great!). The wickedness of sin is owing to the implicit disdain for God. When David committed adultery with Bathsheba, and even had her husband killed, what did God say to him through the prophet Nathan? He did not remind the king that marriage is inviolable or that human life is sacred. He said, “You have despised me” (2 Samuel 12:10). (Desiring God, 58)
Any sinful attitude or action, whether lying or stealing or whether the adultery and murder we see in David’s life, is simply the fruit of sin. But the truest and darkest evil lies in the root of it all—a willful, rebellious contempt for God.