How is sin to be understood? Is it accidental or essential to our humanity? The term accidental refers to those properties of an object that are not part of its essence; they may exist or not exist without changing what that object truly is. For instance, a moustache is an accidental property. If a man shaves off his moustache, he does not cease to be a man.
On the other hand, essential properties are those that are part of the essence of a thing. Remove that property, and it ceases to be that thing. Sin is not essential to humanity, unless someone believes that God made humanity sinful at the beginning. If sin is essential to humanity, then that would mean Jesus was either sinful or not human. So, sin is not essential. Adam had no sin when he was created, yet he was still human. Jesus has no sin, but He is still human. Believers will have no sin when they get to heaven, and they will still be human.
Sin is not essential, but neither is it merely tangential or on the surface of our humanity. Rather, the portrait that we get in the Scriptures of man in his fallen condition is that he is utterly and thoroughly infected by sin in his whole person. In other words, sin is not an external blemish, but something that goes to the very core of our being.