What is the theme in A Severe Mercy by Sheldon Vanauken?

God as providential is the main theme. Throughout the book, with the benefit of Van's hindsight and a mind trained to know the character of God well enough to recognize His hand, circumstances conspire to bring both Van and Davy into intimate friendship with God. The best and earliest example is their shared love of beauty both in human expression and in created nature. When later they are making their investigation into Christianity, the thing that strikes them both so intensely is the beauty with which the Christians interact and worship.