John is a good lad but just the once he steals something.  So he has stolen but that in a sense does not make him a thief.  We mean that being a thief is not part of his character.  His stealing was a once off.  Calling us sinners does not mean we are people who sin but we are sinful people who show what we are by our actions.  Sinner is a reference first and foremost to your character.  It means you don't want God on his terms and you love your sin so much that you won't turn to him on your own which is why he has to help you with grace.  Sinner is another way of affirming the Protestant doctrine of Total Depravity. Its a doctrine that is common in Christianity.  Famously, Calvinism is based on the concept.

Total depravity teaches that we willingly sin and make ourselves depraved and we cannot escape from this in the sense that it is a bad habit we don’t want to break. It teaches we necessarily sin but we don’t coercively sin (page 97, The Institutes of Christian Religion). This means then that salvation is a reward for sinning because it involves God making us righteous not in ourselves but by imputation. It would be different if we were forced to sin but when you sin freely and God does that he is clearly rewarding your sin. The claim that Jesus paid for the sin and so it is not rewarded is just a cop-out. It is like a reward to the sinner therefore it is a reward and is as bad as a reward.

One thing that could have stopped Calvinism and all religions which accepted total depravity from gaining greater popularity is the problem of people who are said to have been unsaved or unrescued by God who cannot find anything bad on their consciences at times. For the doctrine to be correct it is necessary that you sin always until you are saved.

The Bible claims that if you cannot find a sin on your conscience you will find plenty of sins committed in ignorance. For example, if you forget to send the poor money it is a sin or if you have not thought of attempting to convert a bad person it is a sin. God told the Jews that he expected those who inadvertently committed a sin to repent of it and ask forgiveness for it turn him against them for they become guilty of sin (Leviticus 4,5).

God said a person who breaks his Law and does not know it is guilty and shall bear his sin (Leviticus 5:17). Jesus sanctioned this frightening teaching even with his dying breath when he asked God to forgive those who crucified him because they did not know what they were doing (Luke 20:34).

In Acts we read that God did have to ignore the sins committed in ignorance for some purpose but now he expects everybody to repent (17:30). In 1 Timothy 1, we read that God had mercy on Paul for persecuting the Church and cursing Christ because Paul did not know any better. He was forgiven because he was foolish. You are only forgiven by God if you have sinned. So ignorance is a sin. This is how Christians can accuse everybody of being wicked and even the greatest saint of being a sinner. It is reasoned that since saying to a policeman that you didn’t know what the speed limit was won’t get you off it is the same with God. Religion says that there is such a thing as culpable ignorance. If you refuse to learn that something is wrong and then d it then your ignorance does not excuse you. When sin is the supreme evil it follows that no excuse for not knowing is acceptable.
It is naïve to reject this teaching on the grounds that ignorance means that you don’t know what you are doing when you do wrong so you cannot be to blame. You know that neglecting or hesitating to educate yourself in right and wrong and to try and figure out what could go wrong in the day and the right way to handle it is wrong. Ignorance could lead you to cause somebody’s death or some other awful disaster so by embracing it you are really saying that you don’t give a toss about anybody. Your good works are mockeries in that case. Also, anybody you care about is not really cared about – it is the feelings you have for them you are worried about and not them.

Even the anti-Calvinist tome, The Other Side of Calvinism, accepts the claim that ignorantly doing evil is sinful (page 100).

Total depravity condemns our good tendency to do what is best for ourselves and to hell with God. It is a disgraceful doctrine for our desire for independence is a virtue.