We are spiritual beings. Without nourishment for our souls, we are plagued by insatiable cravings—like a body lacking essential nutrients. The spiritual is to us like manna. It is our lifeline, yet we so much fear living from it. Most of us would rather sink our teeth into a steak, or at least a potato—something that feels like a part of our world—than to grapple with a rich but mystifying bread from heaven. But we can't call this life unless we reach beyond ourselves, beyond...