Sin is anything that is contrary to the law or will of God. For example: if you lie, you have sinned. Why? Because God has said not to lie (Exodus 20:16). If you do what God has forbidden, then you have sinned. In addition, if you do not do what God has commanded, you sin (James 4:17). Either way, the result is eternal separation from God (Isaiah 59:2). Sin is lawlessness (1 John 1:3) and unrighteousness (1 John 5:17). Sin leads to bondage (Rom. 6:14-20) and death (Rom. 6:23).
Paul, in the book of Romans, discusses sin. He shows that everyone, both Jew and Greek, is under sin (Rom. 3:9). He shows that sin is not simply something that is done but a condition of the heart (Rom. 3:10-12). In Ephesians Paul says that we are "by nature children of wrath" (Eph. 2:3). Yet, "while we were still helpless, at the right time Christ died for the ungodly." (Rom. 5:6).