It is quite a while since I read Canticle, but isn't there a key element whereby, in this post-nuclear-devastation era, Miller suggests that the human race is starting over, and children are being born without original sin? That would tend to confirm your view, Hitmouse -- of the novel as taking the second approach. My understanding is that Miller was indeed a Catholic, but in the novel he could be using Catholicism as a plot element for a futuristic story that, in its radicality, is questionable as having a Christian theme. But "Crucifixus Etiam" would be sicnece fiction with a truly Christian theme, as I remember it.