The word "sin" appears almost 400 times in the Bible. What is sin exactly? The simplest answer is that sin is disobedience to God. Sin can be thought of as committing a spiritual crime. Most of us do not think of ourselves as criminals. Our tendency is to think of crime in terms of murder, theft, drunk driving, etc. To be sure, these are examples of sin. However, even those of us who have not murdered or stolen are still spiritual criminals (sinners) even if our worst offense is telling a lie! The Bible teaches us that "all have sinned" (Romans 3:23).
Often, we deceive ourselves into thinking that we are good people because we don't commit terrible sins like others do. If we compare ourselves to each other, this might be a valid conclusion. In reality, it does no good to compare ourselves to one another for this is not the standard that God judges by. He does not "grade on the curve". The prophet Isaiah said, "But we are all like an unclean thing, And all our righteousnesses are like filthy rags..." (Isaiah 64:6). In the sight of God, even our good deeds are like something filthy because they are mingled with all the sins we have committed. Tragically, sin of any kind will distance us from God.
What makes something a sin?
We learn in the Bible that the basic concept of sin is the idea of "missing the target". What target is it that God expects us to hit? To answer this, let us look at a question that was asked of Jesus.
"Teacher, which is the greatest commandment in the Law?" Jesus replied: " 'Love the Lord your God with all your heart and with all your soul and with all your mind.' This is the first and greatest commandment. And the second is like it: 'Love your neighbor as yourself.' All the Law and the Prophets hang on these two commandments." (Matthew 22:36-40)
Jesus said that there are two great commandments from God and that all other commandments are based on these two. The greatest commandment is that we are to love God. The second is similar to it; we are love one another. All of the commandments God has given us are based on love. The kind of love which God expects isn't based upon emotion. It is based upon action. It is an expectation which demands that we always act in the best interests of others. We aren't required to feel a strong positive emotional attachment for someone to do what is best for them. If we love someone, we should always be trying to do what is in their best interest even if they are a disagreeable person. If we act in such a way as to put our own selfish desires above the best interest of God or of others, we are not demonstrating love and therefore we commit sin. When we sin we hurt not only ourselves, but others as well.
If we instinctively knew how to love each other, God would not have needed to give us commands and directives. His commands, if obeyed, result in showing love to Him, ourselves and to everyone else. Every command that God had ever given us is based upon the principle of love. His commands aren't meant to keep us from having fun, but rather they serve to teach and protect us. Obeying God is in our own best interest because the commands He has given are for our own good and the good of others. We show our love by obeying all that He has taught us. To disobey is to sin.
All sin is caused by selfishness
The Bible reveals that sin is actually brought about when we act upon selfish desires.
When tempted, no one should say, "God is tempting me." For God cannot be tempted by evil, nor does he tempt anyone; but each one is tempted when, by his own evil desire, he is dragged away and enticed. Then, after desire has conceived, it gives birth to sin; and sin, when it is full-grown, gives birth to death. (James 1:13-15)
Every sin is the result of putting our own desires ahead of God or ahead of others. Every sin is rooted in selfishness. Think back to the example of Adam and Eve. Eve sinned when she gave into her desire to become "wise" by eating the fruit of the forbidden tree. By yielding to her temptation, she not only hurt herself, but Adam as well and ultimately brought pain upon the whole human race.
Some sins have greater consequences than others, but in the end they all result in the death penalty. Sin in any amount severs our relationship with God and that is the greatest consequence of all.