This presumes that the notion of a “religion” is a concept that the ancient Celtic peoples would have had and illustrates a problem in any kind of anthropological research, applying terms from one language and a culture to another and everything getting lost in translation. I don’t believe that there is any one definition of religion either, it depends on which particular school of thought is being followed. So how about not bothering with a bothersome word? Druidism is druidism and if we must needs use a non-Celtic word to describe it, I think a “path” would be the best one – so the “path of those who are as wise as an oak”, i.e. those who are strong in wisdom, which would bring us almost full circle to the concept of a philosophy.