'Tis the season for winged humanoids to alight everywhere from store windows to Christmas tree tops to lingerie runways. But it wasn't always so.

Angels, at least the Christian variety, haven't always been flying people in diaphanous gowns. And their various forms—from disembodied minds to feathered guardians—reflect twists and turns of thousands of years of religious thought, according to an upcoming book.

"There is lots of interesting theology about angels, and in some ways we've kind of lost the knack for that," said John Cavadini, chair of theology at the University of Notre Dame.

"We tend to think of angels as things that we'd find in a Hallmark card," Cavadini added. "But many people, especially in antiquity, were very interested in them"—in what they might look like, how they might organize themselves, how they behave.

In the Bible angels served as envoys of God—angelos being Greek for "messenger." Other than that, the scriptures leave a lot of room for interpretation.

"There isn't a lot of detail, and that's the fascinating thing," said Ellen Muehlberger, a professor of Middle Eastern studies at the University of Michigan.

"The Same Substance as God"

In the early days of Christianity, some believers considered Jesus Christ himself to be one of many angels, said Muehlberger, who's working on a book on the shifting theology of angels in ancient times.

"We only know about this because of later, fourth-century authors who penned negative descriptions of this belief" to refute it, she said.

Jesus officially lost his angelhood when the Roman Emperor Constantine I convened the Council of Nicea in 325. There, bishops were charged with turning the still varied and sometimes conflicting conceptions of God, Christ, and Christianity into a single, unified theology.

"The Council of Nicea defined Christ as totally divine, as of the same substance as God," Muehlberger said.

"Christians who worked to interpret the council's decrees over the next several decades took this to mean that Christ was not an angel. Angels were something else entirely."

A Beautiful Mind

In the early centuries of the church, perceptions of angels may have been as varied as the descriptions of Christ himself—or Judas, for that matter.

A fourth-century Christian monk and ascetic known as Evagrius, for example, developed a theory that explained the human essence in three parts.

"One part is governed by appetites and makes us hungry or sleepy or want to have sex," Muehlberger explained. "That's sort of the lowest part.

"A second is an emotional part that allows us to get angry or makes us prideful.

"Then there is a rational part," she said. "And that is the part, according to Evagrius, that is most like God and the angels too."

Evagrius "thought that something like anger was like a demon that came and attacked you. And if you couldn't fight off those attacks yourself, a totally rational angel, standing beside you, could help you."

Others followed this line, proclaiming that angels were disembodied minds, or intellects, according to Muehlberger.

Angels for Everyone

Around the same time, debate swirled over just who angels served on Earth.

At early Christian monasteries, for instance, many ascetics assumed that really good students would get some kind of divine guide or coach to help them.

"These monks said, Hey, not everybody gets a guardian angel—it's a mark of moral success," said Muehlberger, citing monastic letters from the period explaining the need for monastery inhabitants to cultivate their own angels.

In the towns, though, a more democratic view of angels prevailed.

Bishops and other officials began to assure their congregants that everyone has a guardian angel.

In Egypt, some bishops went on to suggest that some desert-dwelling monks—who had renounced pleasures of flesh and family—might themselves be angels on Earth.

The Egyptian monks rejected this out of hand, saying, in Muehlbergers' words, "We act like animals, not angels."

Eventually this populist view won out: I'm no angel and neither are you, but they watch over all of us.

Celestial Hierarchy

No sooner had believers begun to vaguely agree on what angels were than scholars began to debate how heavenly messengers organize themselves.

The Bible sheds little light on angelic society, but writers have been happy to fill in the gaps, including the unknown author of the circa-A.D. 500 On the Celestial Hierarchy.

Incorporating some earlier ideas, the tome ranks angelic beings into nine orders. From lowest to highest: angels, archangels, principalities, powers, virtues, dominions, thrones, cherubim, and seraphim.

"It was not an official church teaching," said Michael Root, a theologian at the Catholic University of America in Washington, D.C.

Notre Dame's Cavadini added, "I think it contributed to the beauty of the universe that all these different levels of beings were incredibly diverse but completely interdependent, and that all that multiplicity yielded a harmony instead of a dissidence."

Fallen Angels

Of course not all angels are angelic, according to some Christian traditions. Satan himself, it's been said, was once an angel named Lucifer.

The fact that angels can fall from grace is an important point, Catholic University's Root said—it implies that they have free will.

"You even had some theologians in the medieval and the early modern periods who thought that there was an adversarial angel, a fallen angel, assigned to each person as well as a guardian angel—though this was never an official thought," Root said.

As early as the second and third centuries, Christian scholars such as Origen of Alexandria saw important roles for fallen angels, Notre Dame's Cavadini said.

"For Origen and a lot of church fathers, angels participated in the governance of the universe at God's will," Cavadini said.

"That also meant that the fallen angels were intended to participate in the betterment of the universe, and that you have to take them very seriously, because they still did participate—but in a negative way."

Angels in America

Though modern Americans may spend less time puzzling over angels' forms and ways than the ancients did, Americans do tend to believe heavenly messengers are among us, and actively so.

Some 55 percent of Americans think they've been protected by their guardian angels at some point in their lives, according to a 2008 Baylor University survey conducted by the Gallup organization.

"I've been looking at over 1,100 stories we collected from people about their experiences with their guardian angels," Baylor sociologist Carson Mencken said.

"People talk about close calls like auto accidents, especially accidents in which someone else was killed. Others were victims of assault or survived near-drownings or had combat-related near-death experiences," Mencken said. (See "Near-Death Experiences Explained?")

"It's the random death that frightens us—there's nothing that we can do to control it.

"Based on our study, many of the people who survive those close calls attribute their survival to their guardian angels," he said.