Sin - Literally,a missing of the mark, according to the Hebrew and Greek text. God himself sets the "mark" that his intelligent creatures are to reach. Missing that mark is sin,which is also unrighteousness, or lawlessness. (Romans 3:23; 1;John 17:5; 3:4) Sin is anything not in harmony with God's personality, standards, ways, and will,all of which are holy. It may involve wrong conduct,failure to do what should be done,ungodly speech,unclean thoughts,or desires or motives that are selfish.
The "Doctrine of Sin" includes various concepts of sin. Two of the more useful are: Sin as missing the mark---falling short of the goal; and sin as doing not only what is unlawful
according to the standard of men, but of a flagrant defiance of the known law
If you believe in a God and that He has created us for a specific reason, and that He is at least as concerned as the best parent you know about how His children live and come to be with Him in a different mode of existence, then you need the "doctrine of sin" to help you live properly---because the doctrine of sin simply explains your relationship to the God who created you and how you are designed to respond to Him.
So the "Doctrine of Sin" simply allows you to understand your relationship with God and the nature of your interaction with Him.
In other words, it basically shows you what you should be and what you should do based on what you are.
Atheists also have convictions about what you should be and what you should do based on what you are.
Good humans--- Atheist, Christian, whatever---tend to to act the same way in the human sphere---it's that the foundations of our actions come from different principles.
Because of the constraints of the forum I thought it would be best to give you a link that gives the bible's answer to your question.http://www.jw.org/finder?locale=en&docid=502015233&prefer=lang&srcid=share
The doctrine of sin is essential in propagating the Christian religion and in persuading devotees to adhere to church teachings. So "All have sinned and come short of the glory of God" is the premise, followed by, "After death the judgement".
Following from this very basic idea, Christians are then taught that as sinners they need to be saved so that they can spent eternity in Heaven rather than suffer eternal punishment in Hell.
It can be argued that "sin" does not exist outside the religious context. People may be good or bad, caring citizens or crazed psychopaths, but they are not "sinners" if sin has no meaning. Hence, the concept of sin -- not Christ's sacrifice -- is the central tenet of Christianity. Why? Because if we did not need to be saved from something, the sacrifice would not have been necessary.
Of course, when Christians do good deeds because to do otherwise would render them unfit for eternal glory, how do they stack up against non-Christians who do good deeds simply because they are decent, responsible people?
It's an interesting comparison, don't you think?
I hope that helps clarify the question for you.
(Just an aside: If you're going to ask religious questions, don't you think it's a trifle cowardly to do so anonymously? How is it possible to engage in conversation with somebody who won't even identify themselves?)