Halloween is the most vivid contemporary reflection we have of the religious wars that lie at the heart of Western civilization.
It is the most pagan of our holidays.
Its date of Oct. 31 was set by the ancient Celts of Europe as the eve of the first day of winter and the eve of their New Year.
The day recognized the coming of early darkness, the heightened wind, the hard and barren ground, the flight of animals into hibernation.
With the cold and the dark, evil forces that had shunned heat and warmth were believed free to emerge.
The natural boundaries between the human and the spiritual worlds were thought to be temporarily suspended, and humans and spirits mingled. Humans would disguise themselves in masks and robes in the hope that malevolent spirits would think them their own.
The Celts divided their year into quarters, recognizing the important points of the agricultural cycle.
The most important celebration was Samain _ Oct. 31 through Nov. 2 _ which was dedicated to the gods and creatures of the underworld.
Samain was a time of potential danger and enormous uncertainty. The seasons were changing. Life might become better, but might surely become far worse.
To stave off the wrath of the underworld, the Celts built huge bonfires to light the long nights and to purify their animals.
They selected the best of their herds to keep over the hard six-month winter.
They slaughtered the others in sacrifice to the gods and in preparation for a great feast.
They also sacrificed human beings.
So brutal were these Druidic religious rituals that the Celts shocked even the Romans, who came as conquerors.
Despite their high culture, the Celts continued human sacrifice until A.D. 61, when the Roman general Suetonius outlawed the practice and hunted down the Druid priests.
The Druids faded into the mountains and forests that were sacred to their gods and continued the banned rituals down through the centuries.
Meanwhile, with the conversion of large numbers of Celts to Christianity, Druidic practices merged into Christian ritual.
The Catholic church ordered the destruction of idols but often kept the temples, converting them to churches.
It retained the sacrifice of animals, now to be done in honor of saints _ the Catholic version of spirits from the other world.
In the 8th century, Pope Gregory confronted the multi-day problem of Samain by declaring Nov. 1 to be All Saints' Day or All Hallows' Day, enfolding pagan rituals into the church. (''Hallow'' is from the Old English halig, meaning holy person.)
The pontiff declared Nov. 2, the final day of Samain, All Souls' Day, to honor Christians who had died in the faith. This completed the Christian takeover of the holiday.
Yet even as the church honored its saints, descendants of the Celts throughout Europe continued to build bonfires on hills on the eve of All Hallows, now Halloween.
Thus did Martin Luther, with his unerring sense of symbolism, choose this pagan holiday to nail his 95 Theses to the door of the Church of All Saints in Wittenberg, Germany, in 1517.
Among other things, Luther objected to the celebration of All Hallows' Eve, which was going on all around him that night.
For Protestants, Halloween became Reformation Day.
Thus we have paganism, Catholicism and Protestantism all contending over the one holiday.
It was never a benign holiday. As the Celtic scholar Miranda Green wrote, the holiday ''represented a breakdown of normality and order, which were replaced by chaotic imbalance.''
The forces of nature that produced the imbalance in ancient times are still with us (darkness, wind and cold), still producing danger and instability.
Thus we have contemporary horrors like Detroit's Devil's Night, an explosion of brutality and vandalism that occurred every Oct. 31 in the 1980s and early 1990s until halted by the city's current mayor.
The origins of Halloween are never far from its surface. It's well to remember them.