"Is a viewpoint valid, if someone believes in an adult version of Father Christmas?"
I met this comment as a gesture of refusal to even begin to consider anything apparently religious to have any viewpoint to consider or learn from - apart from reinforcing a self-righteousness of superiority in one's own terms.
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Your viewpoint has a validity in that you are no more or less valid than any other human being. That doesn't mean your viewpoint is true, but it means that you are not invalidated by that you are using something and making it true for you at some level.
If you felt the symbol of giving is useful to its reality, you might not need the symbol to do more than work as a focus for intention attention.
If you were to be open to that quality of existence that is a presence and a gift, it may dawn on you in all sorts of ways as a result.
What life is, is something that comes through us and shares itself to then reflects to us.
The symbol of getting is one that takes forms of meaning out of context and redefines to a self-defined context so as to replace art of living with artifice.
So if I said that I still believed in Santa, the fact that I believed it is valid, but not the belief.