Religions that claim universally relevant and abidingly truthful revelations have a clear interest in showing that their history is one of continuity. If you believe that your vision of God and reality in general is in some sense a gift from outside the human psyche, it won't do to allow unlimited adjustments to that vision. But all human language does adjust to historical change, even when trying to stay the same; as Cardinal Newman observed, to say the same thing as your ancestors said, you may well need to say something apparently very different. So how do you resolve the question of what is genuinely an "unfolding" of the original vision and what is an arbitrary elaboration that distorts that vision?

Geza Vermes is the unchallenged doyen of scholarship in the English-speaking world on the Jewish literature of the age of Jesus, especially the Dead Sea Scrolls. In a series of deeply learned and lucid books, he has opened up the subject to non-specialist readers, offering some provocative and searching questions for Christian readers of their scriptures. In this book, he takes the story a little further forward, to trace the evolution of a distinctively Christian vocabulary up to and including the era when the first Christian creeds were being formulated. His subtitle flags up the climax of the story, at the Council of Nicaea in 325, when what he describes as a "revolutionary new formula" was agreed – thanks largely to pressure from a Roman emperor newly sympathetic to the Christian faith, and as eager as any contemporary politician to make it serve the cause of social cohesion.

The shape of the narrative as he tells it is one that most Christian scholars will recognise. In the beginning, Jesus of Nazareth, a charismatic wonder-worker whose profile has some parallels with fairly well-known Jewish saints and sages of his period, proclaims a radically simplified version of the law of Moses and the religion of the Hebrew prophets, with a special stress on the claims of those who think of themselves as having no claims – the destitute, the marginal, the failed. Perhaps the most distinctive feature of all is the way in which Jesus puts at the centre of his world the child, the one who responds without reserve to an unreserved gift of love. "Neither biblical nor post-biblical Judaism," Vermes notes, "make of the young an object of admiration." The early community of Jesus's followers is shaped by charismatic phenomena – healing, prophetic ecstasy – tight corporate discipline, the expectation of the end of the world, and certain social rituals that reinforce the strong family-like bonds of the group. Parts of the family open up to non-Jews, others don't. The language used about Jesus never goes beyond that appropriate to "a man of high spiritual dignity".

What follows is a steady drift away not only from the religion of Jesus and of the first generation but, more seriously, a loss of interest in the essence of "charismatic Judaism" with its suspicion of formalism and its intimacy with God – and an increasingly negative attitude to Judaism as such. The greater the dignity ascribed to Jesus, it seems, the stronger the urge to denigrate and disown his Jewish identity and the Jewish faith itself. With the help of imported mythical, literary and philosophical categories, the Christian community develops a complex system of cosmology in which Jesus has become a co-creator, a pre-existent divine being manifested on earth. It is, in Vermes's words, often a "poetic" achievement, a "majestic synthesis"; but it is undeniably something different from the religion of Jesus and the religion of Jesus's first followers.

I said a moment ago that this is not an unfamiliar account for scholars of Christian origins. It has much in common with the picture elaborated in the great theological schools of the European universities, especially in Germany, from the late 19th century onwards. What makes Vermes's version new is his refusal to follow these earlier scholars in their negativity towards Judaism and in the fact of his unparalleled familiarity with the entire spread of Jewish thinking in the age of Jesus and Paul. His Jesus is very much the representative of an intensified version of Mosaic and prophetic faith, set against a Jewish world that is dramatically diverse and bubbling with new and radical bids for defining Jewish identity.

But some of the themes of an earlier scholarly generation recur. John's gospel has to be treated as a bit of an aberration – though Vermes rightly grants that we cannot write off John's language as simply the result of borrowing from non-Jewish sources. Many, if not most, scholars would be very cautious now about too simple a polarity between "Hebrew" and "Greek" styles of thinking. Again Vermes is inclined to see "Platonic" themes as one of the elements that work the alchemical change in Christianity that will make it unrecognisable to its founder. There is an assumption that the basic alteration is a matter of turning the faith that Jesus himself held into a faith about him.

If Vermes is wholly right, any claims about the "revealed" authority of traditional Christian faith are pretty dubious. The creeds are the product of a very secular chain of political and intellectual influences, serving to obscure the historical core of what was new in Jesus's life and work. But the story is not so simple. Vermes shows how the sort of thing that was being claimed in the creed of 325 had very clear antecedents within a century of Jesus's crucifixion – so that it is odd to speak of a "revolutionary" position advanced in 325. Everyone at the Council of Nicaea believed they were defending immemorial tradition; and they were right to the extent that extravagant language about who Jesus "really" was goes back a long way. Despite Vermes's skilful argument, it is hard simply to deny that Christian scripture does show people praying to the exalted Jesus from very early indeed. Someone reading the convoluted texts of early Christian argument might well see them less as a series of baroque elaborations on a theoretical theme than as a series of attempts to capture an elusive but inescapable insight. Each effort generates more unfinished business; and the impetus is not to clarify ideas for their own sakes but to do justice to the sense that whatever Jesus introduces into the world is new and awkward enough to need a new vocabulary.

This connects with an aspect of the Jewish world of Jesus's day that Vermes skirts a little. The authors of the Dead Sea Scrolls thought of their community in its remote desert setting as the "real" temple. The great structure built by King Herod in Jerusalem was an empty shell: God did not live there but among his faithful servants in the desert. Something of this carries over into the New Testament. Where does God "live"? Among his people; his name, his full, identifiable presence, is to be found among those who associate themselves with Jesus. And if the community around Jesus is now a temple, the high priest who offers sacrifice there is Jesus himself. Put that together with the idea that the earthly temple is a kind of sacrament of the eternal worship in heaven, led by a godlike archangel, and you can see how speculation about who Jesus really is begins to get some purchase. And as this develops into the idea that the angelic high priest really carries in himself the divine name and power, the full-blown doctrine of incarnation, the divine life being clothed in human flesh and blood, steadily takes form.

Vermes's account, for all its lucidity, does not quite allow us to see the energy behind such a movement of ideas. Nor, as other commentators have said, are we helped to see why this particular charismatic wonder-worker rather than others attracted the extraordinary claim that he was the vehicle of unconditional creative power and the enabler of a new kind of worship – the paradox that the creed of 325 enshrined, in words Christians still use. This is a beautiful and magisterial book; but it leaves unsolved some of the puzzles that still make readers of the New Testament pause to ask what really is the right, the truthful, way to talk about a figure like the Jesus we meet in these texts.