A Canticle For Leibowitz

According to TVTropes.org, sometimes a moral leaves no room for subtlety.

A Canticle for Leibowitz is one of those novels. The story revolves around a group of Roman Catholic monks who, after an apocalyptic nuclear war, act as the sole preservers of knowledge and technology in a world where humanity has regressed to the most basic form recognizable as “culture.” The hardest design decision is how much apocalyptic and religious imagery to incorporate. A trinity of bombs, and the simple shape of a papal hat (also referencing a flame and a target) are the main elements in a minimalist design.

Widely considered a sci-fi classic, it has never been out of print since its publishing in 1960.