The ancient Celts believed that in late October, on the night that marked the end of the summer season, the boundaries between the living and the dead blurred and ghosts returned to walk on the earth.

Two thousand years later, the spirit world is just as real and tangible to many people of faith -- and not just on Halloween.

A spirit is a supernatural being or essence, seen in some religions as existing separately from the supreme being, or God. Other religions think of spirits simply as different manifestations of God.

There’s a tendency among skeptics to discredit the belief in spirits, according to Rabbi Bradley Hirschfield, the co-founder and editor of TheWisdomDaily.com. But he cautions against this, saying that the line between the spiritual and the physical is an “artificial contrivance at best.”

“There are forces in the world unknown to us that impact what we do, and the uncovering and taming of them is sometimes done with Prozac, sometimes with talk therapy, and sometimes with prayer,” Hirschfield told The Huffington Post. “They’re all doing the same eternal human thing: Trying to cast out whatever bars us from being what we’re called to be.”

Here are some truly fascinating beliefs about spirits, across a variety of faiths:

1. Spirits exist in many religions around the world.





Indian performers dressed as Asura wait to participate in the 'Ravan ki Barat' religious procession in Allahabad.

The age-old idea that people can be afflicted by otherworldly spirits is prevalent in nearly all cultures, according to David Frankfurter, professor of religion at Boston University and author of Evil Incarnate: Rumors of Demonic Conspiracy and Satanic Abuse in History.

Hindu and Buddhist texts speak of gruesome, fanged asuras and rakshas that can corrupt a person’s soul.

Judaism has never had a richly developed notion of demons, but Jewish folklore does speak of the troubles caused by unhappy or angry spirits. Malevolent spirits called shaydim can wreak havoc in a person’s life and even set foot inside a home. Parts of the Talmud advise Jews to put ashes on the floor of bedrooms in order to capture the imprint of the shaydim’s feet, Hirschfield told HuffPost.

The Quran refers to jinn, invisible beings with fiery personalities who live in the world, but in another dimension. Although the Islamic holy text suggests that some of these creatures can renounce their evil ways, it is believed that most fall prey to their mischievous natures. Jinn share many characteristics with humans -- they are born, they die, and they have the ability to fall in love -- but they can also fly through the air and shape-shift. Many Muslims blame jinn for bad things that have happened in their lives.

In Islam and many religions that originated in Asia, spirits are largely willful beings that can be swayed by their own good or evil desires. People approach them with caution and attempt to determine what the spirit is, what it needs, and why it is present.

On the other hand, in Christianity, these spiritual forces are polarized, according to Frankfurter. Christians believe that the spirits are either the embodiment of all that is good in the world, like the Holy Spirit, or completely evil, like the fallen angel Satan. As delegates of Satan, all demons are sent out to torment and tempt believers away from the path of faith.

Pope Francis has paid special attention to demons, warning his flock to “look out because the devil is present.” Earlier this year, the Latin American pontiff officially recognized an international group of church-sanctioned exorcists -- priests whom the Vatican believes have the power to cast out the devil. The International Association of Exorcists has approximately 250 exorcists working across 30 nations. Francis recently lauded the group for displaying the church's "love for those possessed."

The Rev. Gary Thomas, an exorcist mandated by the diocese of San Jose, says he receives “incessant” calls and emails for spiritual counseling from people all over the world.

Many of these requests are from people suffering from a mental health issue, Thomas admits. But many others have spent years in psychiatric therapy and have yet to find relief. There are real cases of demonic activity, he says, that cannot be explained away.

“We’re dealing with a palpable evil and a reality that is outside of human logic,” Thomas told HuffPost.

2. Spirits are active in the world.





Demons and the damned in hell, detail from Coronation of Virgin Altarpiece, 1454, by Enguerrand Quarton.

For many believers, these demons aren’t just stories found on the pages of sacred scriptures. They are real and ready to make mischief. Apart from the physical realm, many faiths acknowledge the existence of a separate spiritual realm -- for instance, Christian Pentecostals believe in a realm locked in constant “spiritual warfare,” or a battle between the forces of good and evil.

About two-thirds of Americans completely or mostly agree that angels and demons play an active role in humans’ lives, according to a 2007 Pew Research survey. Mormons (88 percent), evangelical Christians (87 percent), and members of historically black churches (87 percent) were especially likely to agree with the idea that these beings are real.

“More liberal Christians don’t take seriously the existence of an evil realm,” says Edgar Lee, a retired minister with the Assemblies of God who is currently a chairperson of the denomination’s Commission on Doctrinal Purity. “But we believe that there is an evil spiritual force that tempts human beings to sinful behaviors.”

Judaism is different. There are mystics in Jerusalem who will write incantations and sell amulets, and the Kabbalah school of thought sometimes speaks of exorcisms. But unlike the other monotheistic religions, Hirschfield said that mainstream American Judaism has never really put an emphasis on the idea of demons.

This may be because many Jews in America are secularized. But it may also have to do with the fact that Judaism is very much a performative religion, full of rituals and requirements, Hirschfield said.

“The essence of Jewish piety is based on a life of law, prayer, study and performance of religious obligations,” the rabbi said. “When things go wrong, the first place you look is not to some external force but to your inside ... and how you are acting.”

Still, Jews still struggle with the same question about evil that many humans face.

“Everyone’s just trying to figure out what it means to believe in one God that is good when there’s so much that is bad in the world,” Hirschfield said.

3. Spirits can attack your body or mind.





Worshippers from a Baptist church perform exorcism on a congregation member in Johannesburg, South Africa.



In some religious traditions, demons are believed to physically attack the body or mind. Members of Pentecostal Christian groups largely believe that demons enter through the mind, tempting people away from the faith.

Some Hindus believe that demonic asuras can eventually take over a person’s spirit, throwing him or her out of harmony with the divine. According to Anju Bhargava, the founder of Hindu American Seva Communities, a person filled with asuras becomes a "raksha" and succumbs to the qualities of anger, greed, and jealousy.

“In our tradition, evil means not having a knowledge of the divine quality within you,” Bhargava said.

Some Muslims believe the jinn can enter and dwell inside the human body before taking it over completely.

The Roman Catholic tradition is very specific about the way that demons assault the body. Demons usually prey on people who are in a vulnerable condition, according to Thomas, the San Jose exorcist.

“They usually choose people who have broken relationships or no relationships, as well as people involved in the occult, which sets off a bell that indicates an interest in a relationship with the devil,” he said.

There are different stages of attachment to the devil, Thomas added: Oppression, obsession, possession, and integration. The last stage occurs when the person has chosen to accept the demonic relationship.

4. Spirits can control people, causing them to behave in terrifying ways.





Pastor Hugo Alvarez performs the exorcism ritual on a woman at the Church of the Divine Saviour in Mexico City, Mexico.



Christian Pentecostals believe demons are responsible for unwanted addictions, like pornography, alcohol and drugs, according to Frankfurter, the Boston University professor.

In the Islamic tradition, malicious jinn are often seen to be the impetus behind mental illness and neurological disease like epilepsy, according to Live Science.

The possession recognized by the Roman Catholic Church results in much more terrifying behavior. There are several classic signs of a demonic attack, according to Thomas. The first is an aversion to the sacred, which reportedly causes people to become physically sick when they come close to something holy, like a church or a communion wafer. People may also have a hidden knowledge of things and possess an inordinate strength. Thomas said he’s seen people burst into Latin, Spanish, Russian, or an ancient, lost language that they’ve never studied. Victims may even begin to shake uncontrollably.

5. Spirits can be cast out.





Jesus and the devil. Detail of elaborate bronze door in the Piazza del Duomo, Pisa.



Exorcism, the ritual casting out of evil spirits, is still widely practiced in America. Frankfurter suggests that the ritual received more attention after the release of the movie “The Exorcist.”



In Pentecostal churches, exorcisms usually occur in a group setting, perhaps during a regular worship service or a special revival meeting. Lee, the retired Assemblies of God pastor, says that these deliverances are performed in “very exceptional circumstances” in which a person feels “unusually demonized.”

“In such a case, the pastor may very well lay his hands on the head of the believer and offer a prayer in the name of Jesus Christ that this person be free of the evil one,” Lee said.

Frankfurter says the experience of an exorcism in a Pentecostal church is an “incredible cathartic experience” for those who may feel they are struggling with certain addictions.

“It’s a social situation that invites this kind of performance,” he said. “I don’t think it’s planned out or deliberate. It’s something people feel called to do.”

In Hinduism, individuals who believe they are being taunted by asuras can pray to the beings and make offerings to appease them, Bhargava said.

Some Muslims also believe that evil spirits, like the jinn, can be cast out of the body. During the ritual, the exorcist reads passages from the Quran, uses holy oils and water, and attempts to speak to the spirit locked inside the allegedly possessed person's body, according to BBC.

In the Catholic tradition, exorcisms are very much individualized. “What makes exorcisms challenging is that every situation is different,” Thomas explained. “That’s the thing about the Catholic tradition. It’s very conservative. It’s not like Ghostbusters -- it’s done with great calculation.”

Thomas said that people come to him privately, out of a feeling of “shame and guilt.” He added that he’s witnessed about 150 exorcisms and has performed between 60 and 80.

An official Catholic exorcist has to receive approval from the local bishop in order to have authority against demons. Thomas was trained at Rome’s Pontifical Athenaeum Regina Apostolorum before receiving a mandate from the Bishop of San Jose to perform exorcisms within the diocese. His training was the subject of the 2011 movie “The Rite.”

To be labeled the victim of a demonic attack, Thomas said the person first has to be cleared by the diocese’s team of psychiatrists and doctors. This allegedly helps to rule out a mental health issue. If the victim’s troubles seem to truly be the result of a demon’s work, Thomas then prepares a spiritual remedy. He attempts to bring the person back into the faith by getting him or her into the rhythm of prayer and sacraments.

A full exorcism, using a complex set of rites and prayers prescribed by the Catholic Church, is only employed as a last resort.

Sometimes, Thomas says, the process is “exhausting” for him and for the victim. And it may take years.

“There’s never just one demon because they operate in a tribe, like lions operate in a pride,” Thomas said. “The demon is usually assigned by Satan and goes out and recruits other demons … And you can’t cast them all out at one time.”

Dr. Valter Cascioli, a psychiatrist and spokesman for the International Association of Exorcists, said that he’s seen evidence of an “extraordinary increase in demonic activity” in recent years.

“We are living in a particularly critical time in history, where urgency, superficiality, exasperated individualism, secularization seem to almost dominate our culture,” Cascioli told Vatican Radio, according to Religion News Service.