I made a list of things that I might consider sin if the concept of sin were valid:
Number 1: Credulity, gullibility. I'd say that's a sin.
Voluntary willful ignorance. I'd say that's a sin.
Letting fear prevent you from understanding reality. I'd call that a sin.
Limiting the rights and freedoms of others in order to make them abide by your standards. That's a sin.
Sacrificing the mental, emotional and physical well-being of a child in deference to your religion. That's a sin.
Wasting the one and only life that you know you're going to have, worrying about and working for an afterlife that somebody told you might be there. That's a sin.