Please can you tell me about the themes C.S. Lewis is exploring in Magian's Nephew other than the religious ones, thanks
There is the theme of good versus evil. This does not have to be a religious theme. Many secular works deal with the conflict of good vs evil in the world.
There is also the issue of being practical versus following the right path. Jadis and Uncle Andrew are practical, and it leads to their downfall. They fail to see the forest for the trees and they miss out on the joy and wonder of Narnia because of it.
Finally, there is the theme that it is often hard to do the right thing, to make the right choice. Diggory could steal the apple and heal his mother, but he knows stealing is wrong and that his mother would not approve. He chooses to do the right thing, and because of this, Aslan gives him the apple anyway to heal his mother, thus rewarding Diggory's right choice.