I think that it's interesting how our readings of the book vary, since it is such an obtuse story, especially for science fiction.

I think what you're missing is how deeply Catholic the view of the world in the novel is is. Excuse me because it's been a while since I've read it, but there is salvation in the book. It's in the very subtle depictions of the Sainted immortal man who appears (implied to be Leibowitz) and in the idea of ascending beyond this world. The world itself, however, is deeply influenced by the Original Sin of mankind, and the pursuit of making the world a good place to live is ultimately hopeless. The monks do right by god but ultimately fail, and their only hope for reward is in heaven. Because the book does not confirm that heaven exists, it becomes a deeply depressing read for people who are not religious, and a story of endless striving and ultimate rapture for those who are. It's a strange and genius way to cause the reader to reflect on their own beliefs.

I think my reading is accurate because the questions of faith are brought up in the book itself in the arguments between the atheist and religious character in one of the generations (that's what I've taken to calling the different parts, lol). I would say the assertion of the book is simply: "This is what humanity's history is. Decide for yourself whether it is meaningless."