Sometimes it seems the harder we try not to sin, the more we find ourselves giving into our sinful nature. For I have the desire to do what is good, but I cannot carry it out. Sin can be defined as directly disobeying God, or as an activity that is offensive. For example, if we pass judgment on people, we are sinning against God. If we rebel against God or our authorities, we are offending God.
Humans are sinful by nature.
With the exception of Jesus, there has never been a person that walked the earth who was not a sinner. In the U.S. Justice system, if someone is found guilty of committing a crime, that person must pay a fine - often a certain amount of money or time in jail. As described above, we all have sinned against God. Since God is just, in addition to being loving, our sin must be accounted for - there is a penalty for that sin. The penalty is eternal separation from God.