We have gone back to the Imperium Truth - Revised Edition
The book that teached something that is not religion and is more a philosophical school of thought.
We all know what the Emperor thought of religion and iconography is tied closely to it. It should be possible to change symbols of what things represent. It doesn't matter what we use as long as they are not already taken as symbols of Chaos have power and the Abomination may cross contaminate if we keep using what are now its symbols.
Now canon says that humans are predisposed and need to believe in something. With this being the major factor why the Emperor's campaign to stamp out belief to put it plainly not very successful.
But where does this say that something people should believe in would be religion or a god? There exists systems that don't fall into any of those categories and could fill the human need for such a thing.
Example I give is spiritualist, Buddhism, some people replace it with science and other schools of philosophy.