You can tell the book was written in the late 1950s or early 1960s because first of all, it capitalizes on the paranoia of mutually assured destruction in a way that later generations never were able to. It comes out at the same time as the movies Fail-safe and Dr. Strangelove, on the heels of the panic that Sputnik caused.

Second, however, and more important from my own personal perspective, is that it shows a religious world that has already been dismantled so thoroughly that it is almost impossible to re-establish it. In the 1950s, before Vatican II, the Catholic Church was a powerful institution unified by a single liturgical language - Latin. Latin was taught in the myriad of Catholic schools that existed in every country, and the Church's influence in many places was considerable. As a result, had a nuclear war taken place in, say, 1959 or even as late as 1962, when the Cuban Missile Crisis took place (The Second Vatican Council would not open until shortly after the end of the crisis), Catholics around the world could rely on the institution of the Church and the unity of language in expressing themselves. Monasteries in many countries were thriving and could resume the functions that they had performed in the Middle Ages (assuming they were located in places that survived the nuclear blasts).

However, Vatican II saw the end of the Latin mass, and with it, slowly, the end of the teaching of Latin in Catholic schools. Many monasteries were closed, and nuns in particular saw a major loss of function and status.

A Canticle for Liebowitz presumes that this whole Catholic superstructure remained in place and survived. I suspect that, if my daughter decides to read the book when she is older, she will find it even less plausible than I did when I read it a few years ago (at Aeryn's recommendation, for the record).