Articles which might enlighten you about the occult nature of Lewis' writings:

http://www.bereanpublishers.com/trouble-in-narnia-the-occult-side-of-c-s-lewis/
http://spaceforswashbuckling.blogspot.com/search/label/Narnia
https://bibleresources.org/witchcraft/

If you cannot understand why a person might warn others of something they've found harmful through experience, I would question your discernment, not mine.

I hope you will understand why luciferianism or any witchcraft is harmful sooner rather than later, before it's too late.