Shorter Catechism Q. 14. What is sin? A. Sin is any want of conformity unto, or transgression of, the law of God.
Commentary: We are not the captain of our own ship. God made us, takes cares of us, and rules over us. We are under his authority, and we are required to do what he says. God tells us what we are to do in his law. The law of God may be found written on the heart of man, which is why people instinctively know right and wrong, and it is found in the Scriptures.
Sin is when we fail to do what God tells us to do. It is to disobey and rebel against God. The reason, for example, it is sinful to kill another human being is not because I think it is wrong or because the one being murdered would prefer to stay alive. It is wrong because God says not to do it. Consequently, it would still be sinful to kill someone even if they wanted you to kill them.
Since God requires us to do certain things and forbids us from doing certain things, we can sin in two different ways. We can disobey God by not doing what he requires. This is what the Catechism means by the phrase, “Sin is any want of conformity unto…the law of God.” The word “want” means “lack,” or “absence.” We sin when our conformity to the requirements of the law of God is lacking. For example, we sin when we refuse to return good for evil by giving our enemy a cup of water when he is thirsty. This is sometimes known as the sin of omission.
We can also disobey God by doing what he forbids. The is what the Catechism means by the phrase, “Sin is…transgression of the law the God.” For example, God says not to steal, yet we go ahead and steal. This is sometimes known as the sin of commission.