I absolutely loved Perelandra.  The imagery and storytelling were entertaining but the most compelling part of me what the battle between the Un-Man and Ransom.

I'm not talking so much about the actual physical fight - which was pretty awesome - but the ongoing verbal struggle the two engage in.  I felt exactly as Ransom did as he tried his best to convince the Green Lady that the Un-Man was lying, that his half truths hid important facts from her, and felt as he did that nothing he could do would be enough.  He begins to wonder why Maleldil hasn't sent anybody to help him.  It was all so unfair.

The climax then is when Ransom realizes that he is who has been sent and that it is his responsibility to stop the Un-Man.  This is the most important moment any of us can have.  When we realize that each of us has been sent to Earth with a specific duty to improve it - in such a way that only we are capable of doing it - we realize the importance of our life, our choices, and our time.  From there every moment takes on new meaning and we find ourselves actively participating in the battle between good and evil.