To understand C. S. Lewis, it helps to read Orthodoxy, by G. K. Chesterton. Lewis, and several like minded British apologists between 1910 and 1960, assumed a foundation of “orthodoxy”.
They were from Anglicanism and RCC, or similar.
They took for granted the basic traditions of Christianity, allowing for variations within that template. They were strong defenders of Reason.
Of course, after 1960, well …
England moved away from orthodoxy earlier than America, thus Mere Christianity was likely popular later here than in England.