Excerpted from "The River of God: A New History of Christian Origins," by Gregory J. Riley. Reprinted with permission from HarperSanFrancisco.

Where did Christianity come from? It went from no adherents in the year zero to become the world's largest religious tradition today. What were its origins?

This is a different question from "Why was it so successful once it began?" It is a question about the historical process that led to its invention. "Invention" is a word rarely used in this connection, but it is an apt word. Early Christians themselves were quite conscious that what they were about was something new. The Synoptic Gospels have Jesus declaring that what he was doing was "new wine" requiring "new wineskins" (Mark 2:22). The cup of the last supper and the eucharist is called the cup of "the new covenant," from which language we get the name for the New Testament in contrast to the Old Testament. The book of Colossians describes the message of the Christians as "the mystery that has been hidden throughout the ages and generations, but has now been revealed" (1:26, NRSV). From the viewpoint of the first-century followers of Jesus, they were onto something fresh, something hidden from the past.

That genuine sense of newness, that excitement about being in the leading ranks of God's new spiritual kingdom, should itself come as some surprise, given the efforts of the early church to claim the opposite. Christians were quite sensitive to the often-voiced criticism that they were a novelty, and for ancients, nothing new was good, especially in religion. A "new man" was an upstart politician from outside the established families; a "new thing" was a political revolution, an attempt to overthrow the state. The Golden Age, like the Garden of Eden, lay in the past, and the world had long since been devolving into chaos. Any religion that had arisen just a few years earlier was clearly something suspect, defective, revolutionary.

Christianity was in fact something new, but it was drawn from and contained ideas that were very old. As a result of criticism about their recent rise on the one hand, and their own need for understanding and legitimation on the other, Christians traced their beginnings back to the Old Testament and cast themselves as the continuation of the history of Israel. Jesus and nearly all of his early followers were Jews, so their own traditions as Jews were their natural background. These Christians saw themselves as heirs to the promises made to the patriarchs, Moses, David, and especially the prophets. They were the new Israel, with roots that went back through old Israel to the beginning of creation itself.

But Christianity in reality had a much wider historical base and a far more complex lineage than the small nation of Israel alone. It is how best to understand that complex lineage is the subject here.

Look at five major subject areas that make up the core of the Christian faith, the main content of the River of God: the rise of monotheism, the subsequent development of Christian Trinitarianism, the arrival of the Devil and ideas about eschatology, the development and consequences of the concept of body and soul for humans, and the advent of Jesus as Savior.

Each of these subject areas had a long history of development. In no case were the initial stages of these ideas anything like what they turned out to be in their final forms in the church of the fourth century CE. In fact, it does nto appear that these concepts existed at all (in the sense that we understand them) at the beginning of the period of our study approximately three millennia earlier: no one we have record of was a monotheist, and certainly not a Trinitarian. There was no Devil, humans did not have souls, and there was no need for a heavenly savior. Nevertheless, there were here and there intimations of each idea--preparations, so to speak, for the eventual rise of the more complex and defined ideas of later times.

In their early histories, each of these concepts was understood differently from culture to culture, and some were completely absent. Each took many centuries to develop and was the result of contributions from several traditions. Inspired individuals and communities of believers melded together competing and often conflicting ideas time and again in periods of crisis. New forms existed side by side with older versions of the same basic ideas. Stage by stage, each progressed, transformed, or regressed to form the eventual Christian doctrines. Over time the ideas became intertwined and interdependent: one first needed a Devil to have an eschatology; one first needed a soul to require a heavenly savior. Even within Christianity itself, when each of these larger ideas was well in place, there was a wide variety of conception from one community to the next. It was not until the time of the creeds in the fourth Christian century and later, when definitions and orthodoxy became prime concerns, that these ideas became standardized. Yet the creeds and definitions bear the signs of long historical development from concepts born in non-Christian societies centuries, even millennia, earlier.