Is it our finiteness? If we fell into this trap because of our finiteness, because we simply weren't smart enough to know the truth, then how would it be just of God to hold us accountable for knowing a truth that is beyond our finite, cognitive ability to know? In other words, how is our unbelief morally culpable if our finite minds couldn't generate the belief we were being held responsible for believing even if we were using that finite mind in the attempt to discover the truth, i.e., we couldn't know the truth even if we wanted to know the truth, because the truth is beyond our finite minds' ability to discover truth? Worse, it seems on this schema our finite minds could generate false beliefs in the pursuit of truth.
Is it because of the fall? Even if it were, then how did our first parents fall such that we could inherit original sin. (And then, if you can answer that, can you explain how we inherit original sin?)