There is an indirect way of knowing what sin is—take the opposite of virtue. In some ancient language, virtue means acting agreeable to or in accordance with one’s original nature and to the society. It is like a flower—it is virtue on its part so to say when it gives fragrance and delights you with its beauty, and sin so to say when it gives you bad odor and looks ugly. Bible too alludes to this when it says: “Put on your new nature, created to be like God—truly righteous and holy.” (Ephesians 4:24) In contrast, when one thinks that his interest comes above all others, it is against his very nature sending bad odor in all directions, hence it is sin. Problem with sin is that it cannot make anyone feel good.
A recent study published in the Journal of Social Cognitive and Affective Neuroscience supports the above. It was found that people who are in love with themselves have poor connectivity between both regions of the brain—medial prefrontal cortex (which is associated with self-referential thinking) and ventral striatum (which is associated with feeling good). That means the more a person feels more important than others and thus deviates into sin, the less he feels good, because of which he will try harder to deviate further creating a vicious cycle. In contrast, real spirituality primarily benefits one’s self and others secondarily.