Sin can be defined as anything that doesn't conform to the holy character of God as expressed in His moral law. The lack of conformity may be in thoughts, in attitudes, or in actions.
Let's consider 5 elements of this definition of sin:
1) Sin is moral evil (e.g., stealing) as opposed to natural evil (e.g., cancer). Moral evil is rebellion against God, and is what brought natural evil into the world.
2) Sin is always, ultimately related to God. Sin is a personal attack on the character and ordinances of God. The central problem of sin is that it offends and incurs the wrath of God. David confessed his adultery and murder "Against you, you only, have I sinned and done what is evil in your sight..."(Ps. 51:4). Sin of course also does have devastating relational, physical, and societal effects. For example, King David's sin did have these consequences against Bathsheba, against her husband Uriah, and against the people of Israel. Yet it is God he has ultimately offended, and it is to God that he must ultimately answer.
3) Sin is breaking God's law. There are sins of omission and of commission. Breaking one commandment is breaking all God's commandments, because it is rebellion against the character of God. "For whoever keeps the whole law but fails in one point has become accountable for all of it." (James 2:10). Compare this verse to Galations 3:10: " All who rely on observing the law are under a curse, for it is written: Cursed is everyone who does not do everything written in the book of the Law." God's unified law reflects His nature and His claims, so rejecting one of His laws is rejecting Him. God does recognize gradations of sin based on Knowledge (Ezekial 8:6, Mt. 10;15, Lk.12:47-48, John 19;11), intent (Numbers 15:30-31), kind, and effect. Yet sin done in ignorance is still sin. Even though God recognizes these degrees of sin on an ethical and human level, all are equally guilty before God and in need of His Son, Jesus Christ, to pay the penalty for sin (to atone for our sin).
4) Sin is deep-rooted in our nature. Sinful actions just reveal the condition of an inner depraved heart. God requires the heart be right (Ex. 20;17, Heb.13:5) not just correct outward actions. These inner attitudes in Scripture are identified as either sinful or righteous. "Out of the heart come evil thoughts, murder, adultery, sexual immorality, theft, false witness, slander (Mt. 15:19 compare with Mt. 7:15-19).
5) Sin has given all of us a corrupted condition and a guilty standing before God. God's pronouncement of guilt is His legal determination that we are in an unrighteous state before Him. The corrupted, polluted condition or state inclines us toward ungodly behavior. But by the grace of God, both his inherited guilt and moral pollution are atoned for by Christ Jesus for " If we confess our sins, he is faithful and just to forgive our sins and to cleanse us from all our unrighteousness" (1 John 1:9).
This grace, mercy, and forgiveness to us through Christ's atoning sacrifice, is a defining difference between God's way and all the human-works oriented religions of the world.
Scriptures to ponder:
I John 2:29, 3:9, 4:7, 4:18 5:1, 5:6
I Peter 2:22
For further reading I recommend:
Jerry Bridges, The Pursuit of Holiness, Colorado Springs: Navipress, 1978, 1989 ff.
Acknowledgement: Much of the ideas are from recent teaching notes of Dennis Wilson.