Sin can be pictured as an archer releasing an arrow from his bow and missing the target. It is not, of course, that failure to hit the bull's-eye in target shooting is a grave moral matter. Rather, the simplest biblical definition of sin is "to miss the mark." In biblical terms, the mark that is missed is not a target filled with straw; it is the mark or "norm" of God's law. God's law expresses His own righteousness and is the ultimate standard for our behavior. When we miss achieving this standard, we sin.
The Bible speaks of the universality of sin in terms of missing the mark of God's glory. "For all have sinned and fall short of the glory of God" (Romans 3:23). To say that "nobody's perfect" or "to err is human" is to acknowledge the universality of sin. We are all sinners in need of redemption.
Sin has been defined as "any want of conformity unto, or transgression of, any law of God, given as a rule to the reasonable creature." In this definition there are three crucial dimensions. First, sin is a lack or want of conformity. It is nonconformity to the law of God. A sin of omission is a failure to do what God commands. If God commands us to love our neighbor and we fail to do so, that is sin.
Second, sin is defined as a transgression of the law. To transgress the law is to cross its boundaries, to overstep its limits. Hence, we sometimes describe sin as a "trespass." We walk where we are not permitted to walk. Here we speak of sins of commission whereby we commit actions prohibited by God. When God's law is pronounced in negative terms, "You shall not," and we do what is disallowed, we commit sin.
Third, sin is an action performed by reasonable creatures. As creatures made in the image of God, we are free moral agents. Because we have a mind and a will, we are capable of moral action. When we do what we know is wrong, we choose to disobey God's law and sin.
Protestantism rejects the classic distinction in Roman Catholic theology between venial and mortal sin. Traditional Catholic theology defines a mortal sin as a sin that "kills" grace in the soul and requires renewed justification through the sacrament of penance. A venial sin is sin of a less serious sort. It does not destroy saving grace.
John Calvin declared that all sin against God is mortal sin in that it deserves death, but no sin is mortal in the sense that it destroys our justification by faith. Protestantism affirms that all sin is serious. Even the smallest sin is an act of rebellion against God. Every sin is an act of cosmic treason, a futile attempt to dethrone God in His sovereign authority.
However, the Bible still regards some sins as more heinous than others. There are degrees of wickedness even as there will be degrees of punishment rendered at the bar of God's justice. Jesus rebuked the Pharisees for omitting the weightier matters of the law and warned the towns of Bethsaida and Chorazin that their sin was worse than that of Sodom and Gomorrah (Matthew 11:20-24).
The Bible also warns us about the guilt incurred from multiple sins. Though James teaches that to sin against one part of the law is to sin against the whole law (James 2:10), nevertheless there is added guilt with each particular transgression. Paul admonishes us against heaping up or treasuring up wrath against the day of wrath (Romans 2:1-11). With each sin we commit we add to our guilt and exposure to the wrath of God. Nevertheless, the grace of God is greater than all our guilt combined.
The Bible takes sin seriously because it takes God seriously and it takes human beings seriously. When we sin against God, we do violence to His holiness. When we sin against our neighbor, we violate his or her humanity.