What exactly is a sin?
We live in a culture where the concept of sin has become entangled in judicial arguments over right and wrong and their possible legal implication and justification. Many consider sin to be violations only of the Ten Commandments and even then there is a tendency to think of 'major' sins such as murder and theft to really matter and therefore be paramount over those such as lying, cursing, or idolatry.
The Greek word for sin is "amartia" which means “to miss the mark.” As Christians, the “mark” or “target” for which we aim is a Christ-like life, one lived to the best of our ability in line with the teachings, precepts, and commandments of Christ. It is the standard of perfection established by God and revealed for us in the life of Jesus.
The Apostle Paul says in his letter to the Romans: "All have sinned and fall short of the glory of God." It's really simple to understand. In the Kingdom of God there cannot be a falling short of the righteousness of God – it is impossible. There is no lying, no cheating, no anger, no malice, no pride, no vanity, no wrongdoing or wrong-thinking of any kind – none. There can only be love and goodness and so viewed in that light it is clear that we are all sinners .
Therefore if we hope to join God in His Kingdom then we must put our faith in Christ alone to get us there. Trying to live good lives by obeying the "law" is not only not enough but is impossible and the Apostle Paul knew that very well.
Therefore we must do our best to live godly lives; to recognise when we go wrong in word, thought and deed, repent of the wrongdoings and then seek forgiveness and put our trust in Jesus who will intercede and take our case before God that we might be forgiven and our wrongdoing forgotten.
If one erroneously believes that there are degrees of wrongdoing then recognise that whether we eat a crumb or a banquet it is still food or whether a drop or an ocean it is still water and so, by the same token, a sin of whatever magnitude is still a sin and is therefore a falling short of God's perfection.