From "A Canticle For Walter M. Miller Jr." by Dave Canfield:
Born in 1922 Miller's childhood is somewhat difficult to mine. He seemed to have no religious upbringing at all even going so far as to call himself an atheist in high school...
While in Italy Miller came into his first contact with the Catholic Church. History does not record all the events that led to Miller's conversion to Roman Catholicism in 1947. But it seems safe to say that the irony of the bombing raid on Monte Cassino was not lost on him as he struggled to make spiritual sense of his life. Miller, in one letter linked becoming a Catholic with his empathy for the Italians although he felt that drawing a direct connection was "after the fact speculation." In any event, in spite of immersing himself in his chosen faith's traditions and rituals his commitment eventually faltered. Unsatisfied he drifted away only to return now and then over a period of about ten years. Said Miller of those days, "by writing Leibowitz, I inevitably maneuvered my head back into the Church. It was an on-again, off-again thing. Finally, I suppose, I tried to define myself in that area by writing [A Canticle for] Leibowitz. So then I went back to the Church for awhile, but it never really took I guess."
Ironically it was after Liebowitz, the writing for which he will continue to be remembered, that Miller grew quiet... [Later he] had long abandoned his Catholicism and in fact responded to Norman Spinrad's description of him as Catholic by saying that referring to him as a Catholic "is as embarrassing for me as it must be for devout Catholics." He was he said still religious though not conventionally so. Yet for a man who wrote so intensely about religion he seemed to draw little in the way of lasting comfort from it. Miller was known to struggle with depression in his later years... A forward for a book of short stories he helped to edit in 1985 showed a familiarity and philosophic sympathy with Zen Buddhism and the Tao Tse Chang and was the last published piece of writing Miller gave us.