Christianity is getting ready for its semi-millennial rummage sale.

Every 500 years or so, the faithful step into their collective attic and sort through what's worth holding on to, observed Phyllis Tickle, a writer and founding editor of the religion department at Publishers Weekly.

"Just as important in this rummage sale is that you're discovering treasures that you've forgotten you have," Tickle said.

Her latest book, "The Great Emergence: How Christianity is Changing and Why" (Baker Books), examines cycles of past and present upheaval.

Tickle is known as an authority on religion in America and a much sought-after lecturer on the subject; her book received recommended reviews from Publishers Weekly and Booklist.

Next week, she'll visit St. Clement's Episcopal Church to speak about the book, and discuss societal shifts affecting the church.

Continuing with her analogy, Tickle said Christians up in the attic may run across a dusty portrait of a least-favorite, long-dead relative in a gorgeous frame. "So, let's cut Uncle Henry out and keep the frame," she said.

"There's an awful lot of fine picture frames being refurbished and dusted off," Tickle said of ongoing shakeups in Christianity. "There's a great deal of Early Church that's being looked at again, both in practice and theology."

Some believers are looking back at the second and third centuries and probing the faith of that time.

An "emergent" Christian is arising from this questioning era, Tickle said. And these Christians are saying, "I want to feel what it was that made it worth going to the lion's den or being burned at the stake, if somebody could just show me what that was."

Their grappling with faith could lead to one that's more communal and less hierarchal — "one that's more inclined to call it the Judeo-Christian story than just the Christian one," she said. "To make it one story and a set of covers, not two stories."

Also, an emergent spiritual hunger — a real belief in what Jesus meant when he said, "If you're not willing to give it all up, you're not worthy of me," Tickle said — could lead to more extreme practices, ranging from radical obedience to neo-monasticism. The latter stresses a prayerful, communal life, focused on generosity and social justice.

Five hundred years ago, the printing press was the "great enabler" of the Reformation and the growth of Protestant faiths, Tickle said. She holds that evolving technology is again acting as a catalyst for religious change.

Also, "emergence," as Tickle calls this time of change, is not limited to the church in the parts of the world where Latin-based Christianity is growing up. The evolution is spurred on by a breakdown in authority and a weariness tied to established social, economic and political institutions.

In the aftermath of Christian history's last shakeup, Tickle said, it took about 50 years for the dust to settle and for Christians to draft a new set of rules and establish who or what was in charge of the church as its leader, or authority.

Of this cycle, she said, "We have to answer the authority question first. I can bet Scripture is going to be one part of it, and community is going to be one part." She predicts that other texts, philosophies and leaders will come into play, as well.

After Oct. 31, 1517, when Martin Luther nailed his 95 Theses to the door of the church in Wittenberg, Christianity changed when his letter was printed and disseminated. During that cycle, the search for authority settled on Scripture as the basis of new rules for the Christian faith, Tickle said.

These days, "that kind of unity of opinion ain't going to happen," Tickle quipped. "Not this time."