Christianity, Islam, Hinduism, and Buddhism together shape the daily lives of more than half of humanity. This book examines these four traditions alongside a fifth: the worship of Zeus, once as widespread and deeply held as any religion practiced today, and now extinct in all but story and stone.

Each chapter traces one tradition from its origins through its central beliefs, its historical spread across continents and empires, how it is practiced today, and its lasting mark on art, architecture, and culture. Yahweh's journey from a persecuted sect to the god of a third of the world. Allah and the fastest religious and territorial expansion in recorded history. Brahman and a tradition with no founder, no single scripture, and no single god, yet the oldest continuously practiced religion on Earth. The Buddha, who denied being divine and is now worshipped as one anyway. And Zeus, whose temples once rivaled any in this book, and whose absence today raises the question this book is built around: what actually determines whether a religion survives.

Grounded in historical scholarship rather than doctrine, the book presents each tradition on its own terms, without ranking which is true. Readers of any faith, or none, will find a clear, comparative account of five of history's most consequential answers to why human beings believe at all.