What is the theme in A Canticle for Leibowitz by Walter M. Miller, Jr.?

Time is a major theme in the book. As an epic tale spanning 18 centuries and dealing with monks under vows of poverty, obedience, and humility, A Canticle for Leibowitz inevitably portrays individual lives as inconsequential. Leibowitz Abbey is responsible from the time of its founding during the dark days of the Simplification with overcoming mankind's determination to forget everything that contributed to the Flame Deluge. For six centuries after the mid-20th-century nuclear holocaust that reduces mankind to the hunter-gatherer stage, the monks indiscriminately preserve anything that falls into their hands, sealing originals in lead-lined, airtight casks in a darkened library.