The Boy Scout camp I attended for two unforgettable summers in the Midwest was set on the shores of a cold, silent, and, at that time, still quite remote lake. Canoeing one day at the far end of the lake, the end nobody visited, I found my way under some overhanging branches into the creek that fed the lake. Slow-moving, surprisingly wide, it opened like a broad, low corridor upon what until that moment had seemed like a closed watery room. What I had known as a lake suddenly revealed itself to be a river.

History itself, until surprisingly recent times, seemed more like a lake than like a river. For a man like, say, George Washington, recorded history stretched back effectively no further than the earliest surviving documents from ancient Greece and Rome--in other words, to just a few hundred years before Christ. The colossal ruins of Egypt were thought to be much older, but how much older no one really knew, because no one could read the hieroglyphic writing that covered so many of their walls. In effect, history was a 2,500-hundred-year lake of time, growing slowly at the near end and not at all at the far end.

The lake of time began to become the river of time when the greatest archaeological find in the history of the West, a trilingual stone tablet found at Rosetta, Egypt in 1799, was deciphered in 1822 by the brilliant Jean-François Champollion. As that epoch-making breakthrough led gradually to the full deciphering of Egyptian hieroglyphic and the extensive translation of ancient Egyptian texts, the West was introduced to men and women whose full, passionate, and even literate lives lay as many centuries before Christ as ours lie after him. The lake of time, instead of growing only gradually and only at the near end, suddenly more than doubled in size, with all the growth coming at the far end.

But however dramatic that quantitative change, the qualitative change was more important still: What had seemed a lake now came to resemble a vast and complex river system. We ourselves were to be located somewhere in the delta where everything flowed bafflingly together.

In "The River of God: A New History of Christian Origins," Gregory J. Riley, a professor at the Claremont School of Theology in California, maps that ancient cultural river system and shows how very much of it, one way or another, has flowed into Christianity. The geographical and chronological reach of this book is staggering: from Persia in the east, past where the Tigris and the Euphrates empty into the Persian Gulf, to Egypt, where the Nile empties into the Mediterranean and westward from there to Greece and Rome. In many ways, this is by now an oft-told tale, but there is something genuinely new about Riley's way of telling it.

Not all that long ago, the phrase "Christian Origins" often meant little more than the study of how the New Testament related to the Old and how the nascent Christian church related to the Greco-Roman culture in which it emerged. Paradoxically, the goal often seemed to be to deny that Christianity owed anything of importance to anybody. Originality was the only boast. Derivation was embarrassment if not impiety.

Riley, against that tradition, finds glory in multiple derivations. For him, Christianity's greatest triumph, historically speaking, is that it has synthesized so much and held it together so well through time.

This process begins in ancient Israel, whose greatness for Riley is not that it so stoutly withstood all alien cultural influences but that it so thoroughly digested them. Israelite monotheism did reject the polytheism characteristic of other Semitic religions, but Israel's God is also in some sense the fusion or assimilation of all the Canaanite, Mesopotamian, and Babylonian gods whom he replaced. Similarly, Israel's creation myth was not an invention from whole cloth but a daring sixth-century BCE Israelite revision of the received Babylonian myth.

Later, in the fifth or fourth century BCE, such now-familiar features of Western religion as heaven and hell, "the" Devil (and his army of lesser devils), and the last judgment were borrowed from Persian Zoroastrianism, borrowings that eventually would become more prominent in Christianity than in Judaism itself.

Still later, there arose the Christian doctrine of the Trinity, which occasions Riley's most fascinating single chapter of religious genealogy. Though this classic Christian belief owes a large debt to the Roman Emperor Constantine, it is fed by a river system of ideas and deities that--as Riley maps it--runs at least two millennia back in time and covers the whole of Mesopotamia, Egypt, and the Greco-Roman Eastern Mediterranean.

In brief scope, Riley has rewritten, for his generation, William Foxwell Albright's 1940 classic, "From the Stone Age to Christianity." But when Albright reaches Jesus, in a chapter entitled "In the Fulness of Time," his book is over. Riley goes on into the history of the church and into difficult theological topics, mostly avoided by his peers in historical scholarship. His own last chapter is a reflective essay entitled "The River of God in the Twenty-First Century."

A wise university president once said that cutting-edge research is best done by headstrong younger scholars, ready to push their crazy ideas to the limit. Introductory courses, he thought, should be taught only by calm senior scholars, seasoned veterans who have learned to distinguished what matters from what does not. "The River of God" is an introduction not to Christ but to the origins of Christianity taught by a senior scholar with a wonderfully large and carefully sifted store of knowledge. Such books don't come along every year. They don't even come along every decade.

My advice? Rent a canoe, paddle up stream somewhere, take this little book along, and then float back downstream reading as you go and thinking about how many the currents are that flow into our civilization. A rich and humane metaphor, the river, and Prof. Riley has written a rich and humane book.