Over here Art Lindsley lists 7 key themes that shaped and are influential for C. S. Lewis.  The 7 ideas are

• Chronological Snobbery
• Desire
• Imagination
• Objective Values vs. Relativism
• Myth
• Immortality
• Comprehensiveness

Two great quotes from Lewis himself, first on desire

Our Lord finds our desires not too strong, but too weak. We are half-hearted creatures, fooling about with drink sex and ambition when infinite joy is offered us, like an ignorant child who wants to go on making mud pies in a slum because he cannot imagine what is meant by the offer of a holiday at the sea. We are far too easily pleased.

And then from the final words of The Last Battle, on immortality

And for us this is the end of all the stories, and we can most truly say that they all lived happily ever after. But for them it was only the beginning of the real story. All their life in this world and all their adventures in Narnia had only been the cover and the title page: now at last they were beginning Chapter One of the Great Story which no one on earth has read: which goes on forever: in which every chapter is better than the one before.