Sin is a religious term that describes a failure to follow a particular set of religious rules. If a person agrees to follow the rules and requirements of a religion, but fails on a certain point, they have committed a religious sin. Only members of a religion can actually commit sin by violating their religion’s set of rules.
Some religious people engage in a bait and switch tactic with the term sin. They will misrepresent the term to mean doing something that is objectively immoral. But when pressed they are forced to admit that they are using the term to mean failing to follow their religion’s rules, as opposed to objectively evaluating the helpful or harmful nature of the action. They are not necessarily being dishonest, they are merely asserting that their theological beliefs are more important than any other consideration.
In many instances not following the prescribed rules of a fundamentalist religion is clearly the moral thing to do in terms of human rights and wellbeing. Following certain ancient world religious customs, by contrast, is engaging in objectively immoral, harmful and destructive actions. Obvious examples would include genocide, slavery, persecution of homosexuals and a lack of gender equality.
Moral religious people have found a way to pick and choose which particular rules and requirements of their religion to follow and which to reject based on the actual affect of the rules. This is sometimes referred to as “cafeteria” religion, but it is an essential part of transforming ancient world religions into a more functional form capable of dealing with the realities of the modern world.