What is the main conflict in A Severe Mercy by Sheldon Vanauken?

Sheldon Vanauken's A Severe Mercy is the story he tells upon reflection of the love he had with Jean Davis, or Davy as she would come to be called, and the evolution they experienced both as a couple in love and as individuals. It is, as much as it is the story of their love, the story of their coming to know and love Christianity and its Jesus. Soon, Davy falls ill, and while she is granted another full year.