Q.14 What is sin? A. Sin is any want of conformity unto, or transgression of, the law of God.
Louis Berkhof writes,
Fundamentally, [sin] is not something passive, such as a weakness, a fault, or an imperfection, for which we cannot be held responsible, but an active opposition to God, and a positive transgression of His law, which constitutes guilt. Sin is the result of a free but evil choice of man. (Systematic Theology, p.231)
So when you speak of “sin” or hear it spoken of in a sermon or Bible study, keep this in mind. Sin is not some mere shortcoming or defect, but either a willful failure to do what God commands (“want of conformity unto” the law of God) or an active doing of what He forbids (“transgression of the law of God”). In short, it is nothing less than rebellion against God and His law.