“And let me make it quite clear that when Christians say the Christ-life is in them, they do not mean simply something mental or moral /…/this new life is spread not only by purely mental acts like belief, /…/ It is not merely the spreading of an idea; it is more like evolution–a biological or super-biological fact. There is no good trying to be more spiritual than God. God never meant man to be a purely spiritual creature. That is why He uses material things like bread and wine to put the new life into us. We may think this rather crude and unspiritual. God does not: He invented eating. He likes matter. He invented it.”

(Screwtape gives advice to use a “body doesn’t matter” philosophy as a way of making the human distracted or ineffective) “At the very least, they can be persuaded that the bodily position makes no difference to their prayers; for they constantly forget, what you must always remember, that they are animals and that whatever their bodies do affects their souls. It is funny how mortals always picture us as putting things into their minds: in reality our best work is done by keeping things out.”

And to your point, it has permeated Christianity more broadly, and arguably western thinking even broader still. I left the church in my teens. My conception of self didn’t shift until decades later after many years of intentional deconstruction. It’s a powerful illusion.