In August 1982, Ronald Reagan’s father-in-law was dying. Nancy Reagan’s beloved dad, Loyal Davis, was an atheist — a troubling fact to the 40th president. So Reagan penned a private, handwritten note in which he recounted how the prayers of colleagues and friends had cured him of a painful stomach ulcer. Giving hope for what lay beyond, Reagan entreated the older man, “We’ve been promised this is only a part of life and that a greater life, a greater glory awaits us … and all that is required is that you believe and tell God you put yourself in his...