What is sin?
A. I John 3:4 says that sin is anomia. Sin is "what is against the law." Sin is conduct or, in some cases, it is a failure to act (James 4:17).
B. Romans 14:23 says that whatever is not from faith is sin.
C. John 8:21 says that you can be in sin.
But to me, the most interesting Biblical concept of sin is when it's referred to as a thing, a kind of embodiment of evil. We see this in the Bible as early as the fourth chapter of Genesis:
"[S]in is crouching at your door; it desires to have you, but you must master it."
What an interesting picture! What is the dark, shadowy, sinister thing crouching at your door, waiting to leap at you? It's sin.
Often we think of sin as something alluring, tempting, deceptive. But sin is ugly. Sin is the costliest thing in the universe. Sin cost Jesus his life.