The latest salvo in the war between the atheists and the believers comes from the doyenne of religious intellectual history, Karen Armstrong. Her tone is one of high-minded irritation. Her argument is compelling. To oversimplify: "faith" and "reason" are not like political parties. You don't join one after having been convinced via argument of its validity. What the Greeks called logos and what they called mythos define two different aspects of the world and our experience in it: the knowable and the unknowable. You can believe in both. The bridge between them, Armstrong submits, is not the snarky badinage or righteous browbeating that has so defined faith-versus-reason debates of late, but practice. By practice she means not the occasional yoga class but genuine, difficult, repetitive practice, which over time gives the practitioner—even the reasonable practitioner—glimpses of the transcendent or the divine. Call it God.

The Case for God, which comes out this month, is Armstrong's 19th book, and it rides the crest of a wave of books meant to dismantle the arguments of the atheists Sam Harris, Christopher Hitchens, and Richard Dawkins. Armstrong is uniquely qualified to write on this subject, for having been a Roman Catholic nun, she then rejected faith. "For many years, I myself wanted nothing whatsoever to do with religion," she writes. "But my study of world religion during the last twenty years has compelled me to revise my earlier opinions…One of the things I have learned is that quarreling about religion is counterproductive and not conducive to enlightenment." Armstrong shows that for most of human history, "faith" and "reason" were not mutually exclusive and that even today all kinds people believe in a God that in no way resembles the God the atheists despise. "Jews, Christians, and Muslims all knew that revealed truth was symbolic, that scripture could not be interpreted literally, and that sacred texts had multiple meanings, and could lead to entirely fresh insights," she writes. "Revelation was not an event that had happened once in the distant past, but was an ongoing, creative process." This critique has not been articulated often or clearly enough: the new atheists are, in effect, buying into one particular modern, Western fundamentalist notion of God in order to make God look ridiculous and knock him (or her or it) down. For them to fail to concede that what William James called "religious experience" is far more complex than what certain contemporary believers preach is extremely disingenuous.

Most provocative is Armstrong's focus on practice—on the activities that help a person engage with God: reading, singing, chanting, meditating, praying, and so on. She has a special affinity for the mystics. The yogi, the Christian mystic, the Kabbalist, the Sufi, the poet—all these, she argues, access transcendence through disciplined work, through failure, anxiety, and the redoubling of effort. By submitting to the unknown, mystics are supposed to become more wise and more loving. At its best, then, mythos has a positive, pragmatic effect on logos. "The point of religion was to live intensely and richly here and now," she writes. "Religious people are ambitious…They tried to honor the ineffable mystery they sensed in each human being and create societies that honored the stranger, the alien, the poor, and the oppressed." It doesn't always work, she adds, but it's worth a try. (Critics will charge that Armstrong's affinity for mysticism leads her naively to overlook the destructive differences among religions. Like Robert Wright, whose recent book, The Evolution of God, argues for a kind of divine morality among humans, Armstrong is more of an optimistist about religion than a pessimist.)

Armstrong's argument is prescient, for it reflects the most important shifts occurring in the religious landscape. In the West, believers are refocusing their attention away from creeds and on practice—on making the activity of faith meaningful in daily life. Examples of this are legion: in the Bay Area, a new school called the Gamliel Institute teaches Jews in every denomination about chevra kadisha, the ancient mitzvah of washing and shrouding a dead body. In evangelical circles, Christians are turning away from salvation talk and toward helping the sick and the poor. Pentecostalism, the fastest--growing brand of religion in the world, stresses the gifts of the spirit: healing, and speaking in tongues. In his new book, The Future of Faith,Harvard professor Harvey Cox calls this new era "the age of the spirit": "Faith, rather than beliefs, is once again becoming [Christianity's] defining quality," he writes. For me, the most refreshing change of all is the possibility, clearly articulated in Armstrong's book, that belief in God requires uncertainty as much as certainty. Sixteen percent of Americans recently called themselves "unaffiliated," a figure that sent religious professionals scurrying for fixes and explanations. But these Americans may just be signaling to pollsters an unwillingness to choose sides.

Miller is NEWSWEEK’s religion editor.

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