Sin is discussed throughout the Bible. It is a huge problem for the entire human race, perhaps the most difficult and most important issue of every generation. It is also the most ignored. The Bible warns us repeatedly that if we remain in our sins we will die (Romans 3:23) and spend an eternity in hell (Matthew 25:41). If sin is such a problem for our immortal souls then we should ask ourselves, “What is sin?”
"Everyone who sins is breaking God's law, for all sin is contrary to the law of God." - 1 John 3:4 NLT
This verse allows us to do a definition by comparison; anything that is against the law of God is sin and anything that is in obedience to God’s law is not sin. So what is God’s law? It is more than just the Ten Commandments. The first five books of the Bible, Genesis, Exodus, Leviticus, Numbers and Deuteronomy are considered the Torah, aka the Law. Even this is not the entire law. Outside of the torah hundreds of commands were given to the people through the various prophets that came after Moses. Thankfully Jesus gives a summary of the law making it much easier to understand, though still impossible to fulfill.
""Teacher, which is the most important commandment in the law of Moses?" Jesus replied, "'You must love the LORD your God with all your heart, all your soul, and all your mind.' This is the first and greatest commandment. A second is equally important: 'Love your neighbor as yourself.' The entire law and all the demands of the prophets are based on these two commandments."" - Matthew 22:36-40 NLT
According to this summary then, to obey the law you must simply love God and love your neighbor. The opposite of these two things, and thus sin, is to love yourself more than God or your neighbor, i.e., selfishness. Selfishness leads to pride, rebellion, disobedience, cruelty, theft; any sinful deed, thought or attitude has its roots in selfish pride. Even Lucifer, the originator of sin, was cast out of heaven because in his pride he loved himself more than God and sought to replace God on the throne of heaven (Ezekiel 28). Temptation to sin has famously been categorized into three groups, the lust of the flesh, lust of the eyes and the pride of life (1 John 2:16), selfishness can be clearly seen at the heart of even these three labels.
If sin is defined as breaking the Law of God how can God be perfectly just and fair in condemning everyone of sin when not everyone is familiar with the Law, some having died before the law was given to Moses? Simply put, everyone is familiar with the law. God has written His law on the hearts of every human being who ever lived, we call it our conscience.
"Even Gentiles, who do not have God's written law, show that they know his law when they instinctively obey it, even without having heard it. They demonstrate that God's law is written in their hearts, for their own conscience and thoughts either accuse them or tell them they are doing right." - Romans 2:14-15 NLT
Breaking God’s law is sin, any selfish thought, action or attitude is sin, the result of any sin is death. God hardcoded His law into everyone through our conscience so that no one will have an excuse for breaking God’s law and we have all broken God’s law. Thankfully God provides a way for our sins to be forgiven through faith in Jesus Christ.
""For God loved the world so much that he gave his one and only Son, so that everyone who believes in him will not perish but have eternal life." - John 3:16 NLT