Glaucon is saying that he doesn't believe in sin, period.
Correct.
And thus:
I know that this is your point. But how does it help people like Glaucon?
How can he avoid the absurd corollary of trying to take precaution against every threat ever made?
I need neither help nor precaution.
My point is that the very term sin (regardless whether one wants interpret sin in the exclusive papauian sense or in a more general fashion) operates out of a greater authority than an individual and their beliefs (much like the narc squad operates in a certain manner regardless whether one believes in it or not)
There is no greater authority than the individual.