‘Canonicity’: the Book or the Reader?

[The Saucepan Man]

I can accept/reject/question the Catholicism that lies ‘behind’ the “Eruism” with the same freedom as in the Waugh novel, but that interpretive ‘layer’ of “Eruism” is always there and unquestionable.

But only to the extent that you are aware of it. And most people's first experience of LotR, say, will be without knowledge of the existence of Eru or even that it is set in a monotheistic world. Eru only becomes "part of the story" as one reads further into Tolkien's works (unless one starts with the Silm). If a person's direct experience of Tolkien extends no further than the Hobbit and LotR, then surely they are entitled to exclude "Eruism" from their interpretation, even if they are subsequently told of its existence.