Are not practices like fasting and self-denial borrowed from earlier or more primitive religions?
I can’t say for certain which bits came into Christianity from earlier religions. An enormous amount did. I should find it hard to believe Christianity if that were not so. I couldn’t believe that nine-hundred and ninety-nine religions were completely false and the remaining one true. In reality, Christianity is primarily the fulfilment of the Jewish religion, but also the fulfilment of what was vaguely hinted in all the religions at their best. What was vaguely seen in them all comes into focus in Christianity — just as God Himself comes into focus by becoming a Man. I take it that the speaker’s remarks on earlier religions are based on evidence about modern savages. I don’t think it is good evidence. Modern savages usually represent some decay in culture — you find them doing things which look as if they had a fairly civilized basis once, which they have forgotten. To assume that primitive man was exactly like the modern savage is unsound.
"Answers to Questions on Christianity," God in the Dock (Eerdmans: 1970) 54.