I used to be a true-believer fundamentalist(-ish) Christian. A big part of what drove me away from that belief system was the absurdity of the doctrine of hell. We believed our scriptures taught that all non-Christians will be tormented forever after death. But the proposed explanations for why a God would allow that are flimsy, while it's totally obvious why someone would make it up. I think belief in hell only remains widespread because we humans are so bad at imagining what it's like to be someone else. It's easy for the believer to feel - at a gut level, even if the Calvinists would consciously deny it - that they chose Christianity because they're more righteous or humble or willing to ask forgiveness than the nonbeliever, rather than because their circumstances and experiences made Christianity look compelling while the nonbeliever's did not. It's easy for believers to say that humans deserve damnation by default and are only saved by the unearned grace of God, but how many would still find that plausible if they found out they were not actually going to receive any such grace? I suspect that, like the Christian in this story who learns too late that Zoroastrianism is the correct religion, most would immediately recognize the unfairness of it.
Another reason people overlook the implausibility of hell is that we can't really appreciate what eternity entails. We're terrible at imagining vast timescales, let alone infinite ones. This book tries (pretty successfully, I think) to give you a visceral sense of what it would mean to live for eons. The protagonist's assignment to explore the Library of Babel illustrates how unutterably horrifying even a mild, boring existence would be if you were forced to endure it long enough.
Unrelatedly, I like this quote from the prologue:
Strange, how a moment of existence can cut so deeply into our being that while ages pass unnoticed, a brief love can structure and define the very topology of our consciousness ever after.