There, in two questions, he poses a problem to young Camilla: if you don't believe in a "personal God," what questions are possible?

For context, note this passage in Tolkien's letters: “The temptation to ‘unbelief’ (which really means rejection of Our Lord and His claims) is always there within us.”

He was socially Catholic, wanted to believe at times, and mostly didn't. He could openly argue for reincarnation and deny the ability of Catholic clerics of any rank to know anything of the afterlife or the nature of souls. = not Christian.