Most religious reformations cast themselves as religious restorations.This makes both logical and emotional sense: After all, if a faith is divinely inspired, then no merely human authority can have the right to alter it. The only way to justify changes in dogma or practice is to claim that they represent a more original, more authentic understanding of God’s will. That is why the most violent ruptures in religious history have started out as movements for renewal from within: think of Martin Luther, who set out to cleanse the Catholic Church of what he saw as abuses by going back to the Christianity of the Gospel. Even a complete maverick like Joseph Smith, when he sat down to write the Book of Mormon, claimed to be completing the Biblical revelation, not contravening it.

These examples are crucial to remember today, when we tend to assume that any believer who claims to be expressing the true, original spirit of his faith is necessarily a reactionary or a fundamentalist.The fundamentals of a religion, history shows, are anything its believers decide they are. When the Unitarians of 19th-century New England transformed their severe ancestral Calvinism into a moderate, philanthropic creed, they believed they were acting in accord with the most fundamental part of Christianity, the ethical spirit of Jesus. Especially in religions based on a holy scripture, like Judaism, Christianity, and Islam, the ability to reinterpret God’s word – to read old texts in a new way – has been the motor of religious evolution, and at times the safeguard of religious freedom.

In her ambitious new book, “The Great Transformation: The Beginning of Our Religious Traditions” (Alfred A. Knopf, 469 pages, $30), Karen Armstrong gives an exemplary demonstration of this kind of progressive originalism. Ms. Armstrong, as readers of her memoirs “The Spiral Staircase” and “Through the Narrow Gate” know, is a former Roman Catholic nun.While she left her order in 1969, her subsequent career as a writer on religion has clearly been a continuation of her spiritual quest, conceived in universal and ecumenical terms. In her best-selling books on subjects ranging from the city of Jerusalem to the life of the Buddha to the history of Islam, Ms. Armstrong has tried to locate in all faiths the spiritual values she regards as fundamentally human.

In “The Great Transformation,” this search has led her to the ancient civilizations where our religious traditions began. The premise of the book is taken from the philosopher Karl Jaspers, who argued that the years from 900 to 200 B.C. comprised a pivotal era in the religious history of humanity, which he named the Axial Age. This period was the seedbed of all the world’s major faiths: It saw the rise of monotheism in Israel, philosophy in Greece, Buddhism in India, and Confucianism in China. It gave the world the Hebrew Bible, the dialogues of Plato,the Upanishads,and the Analects. As Ms. Armstrong writes in her introduction,”The Axial Age was one of the most seminal periods of intellectual, psychological, philosophical, and religious change in recorded history; there would be nothing comparable until … our own scientific and technological modernity.”

The sheer scope of the subject poses important challenges for Ms. Armstrong. She has set out to explain the most profound texts and ideas of four extremely different civilizations, along with the political and cultural history that helped to give rise to them. Inevitably, much of the background she offers feels rushed and cursory: Racing from the Peloponnesian Wars to the Warring States period of Chinese history, and from there to the Babylonian Exile, the reader experiences some world-historical whiplash. More important, Ms.Armstrong gives no indication to the reader that a good deal of what she presents as straightforward history is, in truth, highly imaginative speculation, coaxed by anthropologists from meager evidence. Her accounts of biblical history, or of the prehistorical migration of Aryan tribes, are much less matter-of-fact than they appear to be, and the lay reader would have been better served by some acknowledgement of this.

Another result of Ms.Armstrong’s attempt to cover so much ground is that her interpretation of religious and philosophical ideas can feel too abstract and summary. Indeed, the more one knows about Ms. Armstrong’s subjects, the less fully satisfying her treatment is likely to be. Knowing little about Indian religion, I found her a wonderful guide to a millennium of pre-Hindu faith, from Zoroastrianism to Vedic ritual to the nontheistic spirituality of Buddhism. Of the four major traditions she discusses,it is the Indian, with its emphasis on the universality of suffering and the need for compassion, that resonates most for Ms. Armstrong. “The new hero of the Axial Age,” she writes, “was not a heroic warrior, proudly vaunting his martial prowess, but a monk dedicated to ahimsa [“harmlessness” or nonviolence], who was determined to discover the absolute by becoming aware of the core of his being.”

So much does Ms. Armstrong take Buddhist self-abnegation as her ideal, however, that she has little sympathy for religious figures who do not share it. Chinese religion, with its emphasis on harmony and selflessness, suits Ms. Armstrong fairly well: She writes warmly about Confucius and Mencius. But her other two civilizations, Israel and Greece, can only with great difficulty be portrayed as sharing what she insists are Axial Age values.

Ms. Armstrong is deeply hostile to nationalism; but the Israelite covenant and the Greek polis were both based on a belief that the nation has a particular destiny, a divinely appointed role in history. Likewise, Ms. Armstrong mistrusts scientific reason as a potentially cruel and exploitative force. Yet the teaching of Plato and Aristotle has much more to do with the proper use of reason than with what we ordinarily think of as religion. As a result, Ms. Armstrong is often reduced to reproaching the Hebrew prophets and the Greek philosophers for failing to live up to her ideal vision of the Axial Age. In post-Exilic Israel, for instance, she finds that “suffering and domination had led to a defensive exclusion that was alien to the unfolding spirit of the Axial Age in the other regions.”

It is no coincidence that the two civilizations Ms. Armstrong finds distasteful, Greece and Israel, are also the ones traditionally considered the cradles of Western civilization. Reading “The Great Transformation,” it is hard to avoid the conclusion that the Axial Age has no real coherence – it is too broad a category, covering too much time and space and too many different faiths, to make any historical sense. What Ms. Armstrong is really doing, it becomes clear, is drawing on Eastern religion to critique the deficiencies of Western religion. She looks to India and China for the spiritual resources she finds lacking in her own tradition.That this effort has become so common in the last 50 years, with the popularity of Zen, yoga, and other quasi-Buddhist practices, suggests that there are widely felt inadequacies in contemporary Western religion. By going back to the wellsprings of the world’s faiths, Ms. Armstrong is carrying out a classic reformer’s maneuver: She creates an ideal past in order to find the guidance we need in our troubled present.

akirsch@nysun.com