“For all have sinned and come short of the glory of God.” Romans 3:23
God is simply telling us here that we have all sinned against Him and fall short of everything He expects us to be. This includes both you and me, doesn't it? But let me ask you this, “What does it mean, to sin?”
The Bible tells us that sin is the transgression of the law. (1 John 3:4) God, who created us, has given us His laws, and if we break any of those laws, we sin.
It is something like a chain. If you break one link in a chain, that chain is broken. So, the Bible says that if we keep the whole law and yet offend in one point, we are guilty of all. (James 2:10) You probably have never killed anyone, but have you ever told a lie? If you have, you have broken God's law and are declared a sinner. We can think of many ways and occasions that we have sinned against God, not only in doing what we shouldn't, but in not doing what we should. Isn't that right?
But sin is more than just what we have done. It is what we are, deep inside. God sees our thoughts, motives and desires, and He declares, “There is none righteous, no not one” (Romans 3:10) Jesus told us that if we allow things like evil lust or hatred in our hearts, we sin. ( Matt. 5:23,24)