John 16.8-9 describes, when He comes, He will prove the world to be in the wrong about sin and righteousness and judgment: about sin, because people do not believe in Me.
Most people do not believe they are sinners, people marked by doing bad rather than by doing good. Even with all that is going on today, some say the rioters are not sinners, just expressing righteous indignation, others say the police are not sinners, they are just forcibly dealing with bad people. No one believes they are a sinner.
We define sin as murder, rape, stealing, maybe lying, cheating. All things we have all done mentally if not physically, yet not routinely, therefore in our minds we are not sinners. While God counts all these things as sin since He made laws commanding we never do them, these things are not The Sin that will keep us from heaven and reconciliation with God.
The sin that keeps us from knowing God and pleasing God is the sin of ignoring God Who came to us in person as Jesus the Savior of the world. When we say, or allow others to say without rebuttal, all gods are the same and lead to the same destination, we deny Jesus as the only True God, then we sin. When we decide how we will live to please God, making our own way to heaven, we deny Jesus as the only way, and so we sin.
Sin is believing and living as though Jesus didn’t exist or that what He says is irrelevant for our lives. The sin that God will condemn us for first, most and only, is the sin of not recognizing and therefore obeying, His presence upon earth as Lord and Savior in Jesus, the ‘carpenter’ from Nazareth.
God has come to us to teach us, show us and to provide for us reconciliation with God the Father but if we refuse Him we commit the sin that will keep us from living with Him and all those who have ever loved Him, forever.