What is Sin?

A friend of mine has a talk/paper to give on sin today. He randomly asked me if I had any resources lying around that might help. Below are some quotes I sent him.

Certain new theologians dispute original sin, which is the only part of Christian theology which can really be proved - G. K. Chesterton (Orthodoxy, chap. 2)

The 7pm project on the 22nd Feb had a story on school bulling where one of the panelist asked a child psychologist if bulling comes from original sin, to which she denied. You can watch that here (around the 10:45 mark).

Something with some theological words in it:

The problem of our legal guilt and condemnation before God is solved by his reckoning to us the righteousness Christ; and the problem of our moral defilement and habitual sinning is solved by his purifying us by the work of Spirit. The first remedy, justification, comes by imputed righteousness. The other, sanctification, comes by imparted righteousness. Justification is instantaneous; sanctification is progressive - and we will deal extensively with it in Romans 6-8, just we have dealt with justification in Romans 3-5. - Desiring God Staff from here.

Something that might need more thinking about (or that I am unsure of):

If Adam and Eve could not sin apart from divine providence, and if they did sin under that providence, we must conclude that in one sense it is good that they sinned. The sin itself was not good, of cause, but allowing and ordaining it was part of God's will, and he wills only that which serves his good purpose. - R. C. Sproul (Truths we confess Volume 1, p159)

It can no longer be denied that God, in principal, has willed sin - Some author I didn't write down (a big book called "Sin" that is in my church library, p60)

And some "textbook" definitions:

So what is sin? Sin is a violation of the law of God. It's a violation of the moral character of God and His laws as revealed on the pages of Scripture...that is sin. That's all we need to say about it...that's all we need to say about it. - John MacArthur (What is sin?)

Sin is any failure to conform to the moral law of God in act, attitude, or nature. - Wayne Grudem (Systematic Theology, p490)

Sin is anything (whether in thoughts, actions, or attitudes) that does not express or conform to the holy character of God as expressed in his moral law. - ESV Study Bible

Sin includes both omission (where we do not do what we ought) and commission (where we do what we ought not do). Sin includes our thoughts, words, deeds, and motives. Sin includes godlessness, which is ignoring God and living as if there were no God, or as if one were God. Sin is invariably idolatry, which is the replacing of God with something or someone else—most often 
oneself. - Mark Driscoll (Fall:God Judges)

Driscoll is the only one that mentions the sin of omission in his definition, the others have omitted it...