Now, as to that theory of history. I think it can be pretty well summed up by reading Genesis 2-3, where the Lord forbids man the fruit of the tree of the knowledge good and evil. Because man eats of this tree, he is forbidden the fruit of the tree of life. A little knowledge proves in this case to be a very dangerous thing.
Canticle starts after a nuclear war, when the price of man's increased knowledge has been obviously disastrous. The first novella ends with one of the monks of St. Leibowitz getting killed in the wilderness after dedicating his entire life to preserving knowledge he doesn't even understand. The second novella ends with another senseless death, this time a poet killed by resurgent savages caught in political machinations by a larger state trying to rule with technology. The third novella ends with the world once again having the capability to start a nuclear war -- and using it.
Miller does throw in a ray of hope. Some of the members of the Order of St. Leibowitz escape into interstellar space, with enough bishops to ensure the apostolic succession. They're on their way to the struggling colonies on other planets. Yet I think this is not the hopeful ending that it might at first appear to be: the monks take with them a copy of the memorabilia of the order, the very knowledge that's already doomed mankind on his original planet twice over. Miller seems to be saying that our moral sense will inevitably be overwhelmed by our monkey curiosity, and that we're powerless to avoid taking the seeds of our own destruction with us anywhere we go. Individual people might be saved by faith, but the race as a whole will continue to live out its days in anger and misery.
Despite this downbeat worldview, the novel is well worth reading. Miller has the knack of making the future world seem real with an odd detail here and a throwaway reference there. It's too bad that this is the only major work we have from him.