This is a very important question, and while the answer might be simple, the effects of sin are profound. It would be wonderful if there were no such thing as sin. It would be fantastic if every human being was by nature loving, patient, kind, and full of goodness. But that isn’t our reality. 

Sin, at its core, is living apart from God. Speaking through the prophet Jeremiah, God said, “My people have committed two sins: They have forsaken me, the spring of living water, and have dug their own cisterns, broken cisterns that cannot hold water” (Jeremiah 2:13). Sin, in God’s definition, has two parts, (1) we turned our backs on the God who created us, and (2) attempted to exist on our own, apart from God. Every other sin proceeds from these two choices.

It is also important to note that when the Bible speaks about sin, it not only speaks about sinful acts, but it also speaks of sin as a power. Sin is not only the act of lying, cheating, stealing, or some other wrongful act. Sin is the thing in us that compels us to do wrong. The apostle Paul wrote the clearest description of the battle within every human. 

“We know that the law is spiritual; but I am unspiritual, sold as a slave to sin. I do not understand what I do. For what I want to do I do not do, but what I hate I do. And if I do what I do not want to do, I agree that the law is good. As it is, it is no longer I myself who do it, but it is sin living in me. For I know that good itself does not dwell in me, that is, in my sinful nature. For I have the desire to do what is good, but I cannot carry it out. For I do not do the good I want to do, but the evil I do not want to do - this I keep on doing. Now if I do what I do not want to do, it is no longer I who do it, but it is sin living in me that does it” (Romans 7:14-21). 

In summary, sin is rebellion against God. It is not merely doing bad things, but rather it is a power that compels people to reject God.