Despite the rain and the fact that we're standing in the middle of nowhere, in the dark, on the semi-rural outskirts of Brisbane, and despite the unexpected arrival of the police and the reality that the "altar" is actually just a tree, I'm pleased to report that my first officially certified Satanic ritual goes pretty much to plan.

According to the 2011 census, there are now more Satanists in Australia than Scientologists. This could be seen as cause for alarm, until you actually meet some Satanists, who, judging by the ones I'm standing with right now, are only marginally more intimidating than my daughter's primary school debating team. There is Sarah, a startlingly pretty 30-year-old from the Sunshine Coast, and Todd, a tall, 33-year-old anaesthetist's assistant. There's a guy called Johnny, who chain smokes, another guy called Jim, and Mark, aka the Warlock Marquis HK, who is our MC for the evening. The Marquis is solidly built, with osteoporosis of the hip and the kind of speaking voice you might expect to hear selling hair-replacement technologies on commercial radio.

The Marquis has brought us to this secluded spot in Moggill, west of Brisbane, to celebrate Walpurgisnacht - April 30 - a date of some significance in the pagan calendar. Though it is rather confusingly named after an English medieval saint, Walpurgisnacht is believed to be the night that a bunch of witches got together, several hundred years ago, on a mountaintop in northern Germany. Ever since then, it has been a red-letter day for wiccans and warlocks everywhere, an occasion upon which it is deemed good and proper to get together, catch up with old friends and offer some prayers to the Dark Lord.

As for the location, the Marquis is nothing if not practical. "I live with my folks," he says. "There's no way they'd have this happening at their place.”

Notwithstanding the absence of a sacrificial human infant or even, say, a chicken, the altar is impressive, a small wooden table propped against a tree and draped with a red-silk cloth. On top of the cloth is a selection of ceremonial objects, including a statuette of Kali, the Hindu goddess of death; a dagger; a stone skull; a figurine of Baphomet, the Sabbatic Goat; and a Viking buffalo horn chalice. ("The buffalo wasn't killed for the horn," Todd assures me. "I bought it online.") There are also some candles - black candles to the left, a single white candle to the right - and a copy of The Satanic Bible, by Anton LaVey, the American occultist and founder of the Church of Satan, to which everyone here belongs. Hanging over the table, from the branches of the tree, is a banner featuring the Sigil of Baphomet, a pentagram that has long been the Church of Satan's official insignia.

The Marquis begins with the Invocation to Satan: "In nomine Dei nostri Satanas Luciferi excelsi!" he intones, channeling Vincent Price via Logan Valley. "In the name of Satan, the Ruler of the earth, the King of the world, I command the forces of Darkness to bestow their infernal power upon me!" He then lists some of the many names of Satan ("Mammon ... Shaitan ... Lilith ... Asmodeus ..."), before summoning the four crown princes of Hell: "From the South I summon thee Satan Infernal Majesty, Come forth, we bid thee welcome! From the North I summon thee fearsome Belial, Come forth, we bid thee welcome! From the West I summon thee dread Leviathan, Lord of the Abyss ...”

At each summoning, Todd turns to the cardinal points of the compass, saluting the starry firmament with a large sword he has produced from the boot of Sarah's car, taking care not to decapitate anyone. The Marquis then reads a blessing, the gist of which I can't quite catch, before impaling the paper it is written on with the dagger and setting it alight with the black candle. This is quite dramatic. Unfortunately, our photographer wants him to repeat the gesture, which proves difficult, as we have run out of blessing paper, and the scraps that remain are 90 per cent burnt and somewhat wet from the rain. Though the Church of Satan does have a minimal protocol for effective rituals, the most important thing, as the Marquis explains later, is "desire. That's the bottom line. If the desire is there, you can kind of improvise the rest.”

And so, with blessings burned and maledictions muttered, the Marquis passes around the Viking horn chalice, which is filled with tawny port, and everyone has a swig. And that's that.

Or so we think.

Just as we finish packing everything in the car - the sword, the dagger, the stone skull - a set of car headlights appear. A policeman emerges and approaches us, shining a torch in our faces. "Hi guys," he says. "Whatcha doing out here at this time of night?" Time freezes. I look at my companions, who now actually look quite scary, dressed, as they are, entirely in black, with their hair plastered wetly on their faces, standing in the middle of the bush late at night for no discernible reason.

"Oh, we're Satanists!" replies the Marquis.

Just joking. He does nothing of the sort. Instead, he pauses and, with the composure of a man well practised in answering awkward questions from authority, says: "We're musicians. We're just shooting an album cover.”

"Right," the cop says.

"We're leaving now.”

"Uh huh," says the cop, who I sense is in no hurry to hang around. "Well, don't let the council catch you here. They could fine you.”

"Oh, okay, thanks.”

The cops take down Sarah's number plate, just in case, before departing. The Marquis turns to me. "We're having a barbecue now. You're free to join us, if you want.”

Satanism is a broad church, with a congregation to suit almost everyone, from law-abiding occultists to crypto-Nazis and authentically psychopathic devil worshippers. As with Christianity, this is a supermarket of spirituality amply stocked with churches and microchurches, the names of which - the Black Goat Cabal, the Temple of Set, the Ordo Flammeus Serpens - speak, if nothing else, of a particular flair for branding. The biggest of the lot, however, is the Church of Satan, a group founded in 1966 by the aforementioned Anton LaVey.

Born in Chicago in 1930, LaVey was a talented musician who left school at 16 to join the circus. After moving to Los Angeles, he worked as an organist in bars, nightclubs and revival halls, an experience that fed his disenchantment with religion, whose adherents, he noted, sought a good time on Saturday night and absolution on Sunday morning. He had always dabbled in the occult and claimed to have worked as a "psychic investigator" for the San Francisco Police Department. In 1966, at the urging of a friend, he formed the Church of Satan, shaving his head and proclaiming 1966 as "Anno Satanas", year one in the age of Satan. Three years later, in 1969, he published The Satanic Bible, a collection of ruminations and rituals that is now considered the essential text for LaVeyan Satanism.

LaVey promulgated what is known as "atheistic Satanism", a philosophy in which the Dark Lord serves not so much as a literal figurehead but as a symbol of pride, liberty, and individualism. "We are the first above-ground organisation," says the Church of Satan's website, "dedicated to the acceptance of Man's true nature - that of a carnal beast, living in a cosmos that is indifferent to our existence." Hedonism is a birthright, and indulgence is endorsed. Sexually, anything goes, so long as nobody gets hurt. "A lot of religions put guilt on you for having human desires," Sarah says. "But this is empowering. It's about honoring your instincts.”

It's also very Gordon Gekko: greed is good, ambition is awesome, stupidity is a sin. LaVey was criticised for his social Darwinism ("Death to the weakling, wealth to the strong," he wrote), but Todd says this can be misconstrued. "We are not like Nazis," he says. "Satanism is just about being the apex of all you can be, without clinging to excuses." In its super-charged egoism - the highest holiday is one's own birthday - LaVeyan Satanism actually resembles the kind of two-fisted libertarianism you see in investment bankers and corporate lawyers. (LaVey was heavily influenced by Ayn Rand.) Lastly, it is most definitely not about devil worship; indeed, the whole idea of worshipping other entities is nonsensical, as you are your own god.

Todd was 20 when he discovered Satanism. "I always looked for alternative avenues of thought," he explains, leading us to a public barbecue area. "Then one day I found a copy of The Satanic Bible on the coffee table of a friend. I read it, expecting a whole bunch of rabble rousing and anti-Christian antics. Instead I found it to be quite grounded and real. I thought, 'Wow, this has been me all along.' " Satanism was a passport to a parallel realm of inquiry. "It made me rethink my values on freedom, and place more emphasis on the 'freedom to' rather than 'freedom from'," Todd says.

Todd, like everyone here, is a paid-up member of the "CoS". "There's a joining process," he says. "It's mainly to weed out devil-worshipping crackpots and people like Richard Ramirez." (Ramirez, aka the Night Stalker, was an avowed Satanist who murdered 13 people in California in the mid 1980s.) There is also a $200 joining fee, which buys you a silver Baphomet pendant, a black manila folder with a 'Welcome Aboard" letter, and a personalised crimson business card, underneath which is written: "Citizen of the Infernal Empire". Other products can be purchased online from the CoS emporium. "The Church of Satan is a business, just like all other churches," Todd says. "The only difference is, the CoS admits it.”

In 1980, a Canadian woman named Michelle Smith published a book called Michelle Remembers. In it she recounts how, as a young girl, she had endured years of Satanic ritual abuse (SRA), including rape and torture, spending hours naked in a snake-filled cage, witnessing babies being butchered, and being forced to defecate on a Bible. Michelle Remembers was co-written with Smith's psychiatrist (and later, husband), Lawrence Pazder, who blamed much of the abuse on the Church of Satan - assertions that were later withdrawn after LaVey threatened to sue. Despite this, and the fact her book was exhaustively discredited, Smith's story sparked a wave of similar allegations, fuelling what became known as the Satanic Panic.

For connoisseurs of conspiracy theories, the Satanic Panic is the big one; the white whale. Satanists are, apparently, all around us, in secret cliques and underground networks of otherwise upstanding people who in their downtime just happen to engage in murderous orgiastic rituals and child-sex abuse. While researching this story I was contacted by a Christian group called Love For Life, the leader of which assured me that at least 500,000 Australians have been affected by Satanic sexual abuse, "often involving figures in high places, all condoned and kept secret by police, judges, church figures, politicians, bureaucrats/public servants, experts, etc, etc, many of whom 'sold out' to be part of these Satanic networks.”

I try to imagine the Marquis catching up with, say, a High Court judge to crap on a pile of Bibles and sodomise street urchins, but it's difficult, because he is, basically, rather nice. A bit odd, perhaps, a little maladjusted socially, but polite, even sweet, in a slightly Gothic, death metal type of way, something that may or may not have to do with the fact that he comes from a family of devout Christians. "They know I'm into Satanism, but I don't talk about it with them.”

The Marquis is a keen writer, and for a short time published a Satanic lifestyle magazine called The Sentinel. He is working on an erotic comic book series called Slices Of Sin, and has recently finished a memoir, Thirty Years Of Anger, which he describes as "an autobiographical account of my journey through the Australian hardcore punk and underground extreme metal scenes". For money, he does casual security shifts. For leisure, he does beer, bands and barbecues. He is a collector - horror toys, comic books - and an avid traveller. "I love scenic places, mysterious places, and being surrounded by nature." He is, in other words, a romantic. A sceptical romantic, which sounds like a paradox, but a romantic all the same, with a corresponding faith in magic. "Rituals are very important," he tells me, unpacking a bag of sausages. "The magic is real for us. We believe in it.”

The Satanic Bible devotes an entire chapter, "The Book of Belial", to the proper practice of magic, and is very specific on the necessary preconditions regarding dress (black cloaks, preferably hooded, so as to allow participants to express themselves unselfconsciously), the altar (ideally a nude woman, head pointing south, feet pointing north), and a gong (to be struck once after the participants have repeated after the priest "Hail Satan!"). There is also mention of phalluses, chalices and voodoo dolls. Rituals falls into three main categories: lust, compassion, and destruction, but practitioners often experiment. "Last year I did a spell to get a job," says Johnny, who has started drinking Wild Turkey. "And it worked.”

The last spell that the Marquis did was to save his pet dog, Bill. "Bill had kidney stones," he says. "The vet told me it was really bad, and I said, 'bollocks!'" The Marquis performed a solitary, and then a joint ritual, held remotely but in conjunction with a Satanist in Melbourne, called Veronica Black. "I got a photo of Bill, and a bit of his hair, and burnt them both in the flames of a black candle while she held one of her puppies, drawing all the young dog energy and focused it on my dog." Within a week, Bill was running around. "You should have seen the face of the vet when I went to order Bill's food," says the Marquis. "He thought he would have been dead by then.”

The rain that soaked the ritual in the bush has become heavier, and we huddle underneath a shelter. Todd is struggling with the hotplate, which isn't getting hot, because it's being rained on. The barbecue might have to wait. In the meantime, more cigarettes are smoked, more Wild Turkey is drunk. But I am hungry. I consider asking them to perform a Satanic "No Rain" ritual, but think twice. In the end, I say goodbye, and head back to the city, stopping at a McDonald's for a wickedly delicious Big Mac.