The answer is found in Isaiah 59:2:
"But your iniquities have separated you from your God; your sins have hidden his face from you, so that he will not hear." (NIV)
A discussion of sin is very important to anyone who has a belief in God. If we accept the existence of a Creator, our next major concern is pleasing Him. Consider ... an Omniscient, Omnipotent Being is responsible for our existence. Our continued well-being is totally dependent upon His whim. Is it not in our best interest to determine what He desires of us, what is acceptable and unaccetable to Him?
Sin, put as simply as possible, is that which is unacceptable to God. It is what separates man from God, and from the possibility of ultimate happiness. Since the Bible is the Word of God, it is logical to go to it to find out about sin. This is an important initial point. If we want to know what sin is, and what things are sinful, we must go to God to find out...NOT MAM.
For example, many do not believe that dressing immodestly is sin. Others go so far to think there is nothink sinful about lying, getting drunk, gambling, or cursing. Abortion, Homosexuality, Racism, etc., you can always find someone who will deny such things are sinful. But it is important what man may think about such things. The question must be, "What does the Bible say about sin?" It is only asking this question that we can know what God considers sinful. And it is only in knowing what God considers sinful that we can ensure we are pleasing Him.