The Ceremony

The Ceremony is a religious ritual performed by the Hymn of One and the Order with a Trait Positive girl. The Hymn of One's ritual acts as a diguise for the true ritual, performed by the Order.

Hymn of One
Bree is honored from the start that she was chosen for the Ceremony, but never explains how this happened. She eventually learns from her father that the Order selects a pool of young girls who show "potential" at birth. These children are placed in what they call "protected homes" with adoptive parents from within the Order. When they reach puberty, they single out one to participate.

The Order
The true criterion on which girls are selected is whether they are "Trait Positive", meaning their counts of a special ribozyme are exceptionally high. If that is the case, the girl is abducted as a baby, and placed with parents from the Hymn of One (under the above-mentioned disguise).

Hymn of One
The religious disguise for the Ceremony, the supposed purpose, was never revealed in the series - mostly because Bree didn't know it herself.

The Order
The true purpose of the Ceremony is to transfuse the blood of a Trait Positive girl into an Elder, to prolong his life for an additional 40 - 50 years. The rogue Elder claimed in What Happened? that, while he looked "alright" on the outside, his liver and kidneys were ninety years old.

Hymn of One
For the religious disguise, Bree had to undergo rigorous preparation for the Ceremony. She had a special diet, took iron supplements, received injections of a then-undisclosed substance, learned the rare language of Enochian and practiced exercises and walking steps with her new helper Lucy. The reasons for many aspects of her preparation remain unclear. Her injections and pills in particular had sparked curiosity in many viewers. However, it should be known that the injections were actually used to make Bree trait negative. The iron pills, however, are still a mystery.

In season 2, Emma tells us she is preparing by learning a dance that "some people do find weird", as well as "a chant that needs to be recited". Emma, contrary to Bree, talks about being "special" because of her blood, and does know about the blood transfusion procedure - she claims it is called "chelation" and supposed to "build up the health of my blood and strengthen my spirit". She also claims that, after the Ceremony, the Ceremony Girl is taken to a "special place in the mountains, near Sedona" where she "learns all the hidden secrets of the universe".

One should keep in mind, though, that Emma had seen the video of Bree's demise (including the transfusion), and learned about blood traits, ribozymes etc. from Jonas and Daniel. It is thus likely that this is information not usually given to Ceremony Girls, but in fact a unique cover-story to let what she saw and heard look non-threatening and misinterpreted, and to give a reasonable explanation why Bree hasn't been seen since her Ceremony.

The Order
For the true purpose of the Ceremony, the blood transfusion, most probably only the injections and pills were important, to further boost her ribozyme levels. This view is supported by the rogue Elder in What Happened?, when, replying to Claire's fear Emma isn't ready yet, he says: "Her ribozymes are up at trait positive levels. Of course, she's ready." - clearly implying that nothing but the ribozyme-levels matter.

Hymn of One
Judging from Bree's accounts of the Ceremony, it is held rarely. She and her parents considered it a great honor that she was selected to participate in it. Her parents themselves were not invited to attend, although they did greatly encourage her participation. Not even Bree knew exactly what the Ceremony would be like. She imagined that she would have to do the walking steps and use her knowledge of Enochian, but other than that she did not know what to expect.

The Staged Ceremony
On October 12th, 2006 a fake Ceremony was held and Bree posted a video soon thereafter happily stating that it had been a success. Daniel had followed her to the site of the Ceremony and filmed what he could see of The Ceremony. It was held outdoors at night in a wooded area and little could be seen clearly except some dimly lit hooded figures, among them was Bree. A few days later Daniel sensed that something was amiss, and eventually Bree confessed that the Ceremony was not the real one, and that she had been persuaded to lie to him about it. The Order had taken the opportunity to photograph Daniel spying on them, and Bree was remorseful when she found out.

The Order
From hour ten of the season 1 finale, it appears that the true Ceremony is nothing more than a method of extracting trait-positive blood from young women, a normal patient-to-patient blood transfusion. It was "held" with no special celebration in an underground medical complex owned by the Order, however, in the season 2 episode Spreading the Song, Sonia asks Carl "Have you even seen Bree since the Ceremony?" - which can be understood in a way that implies that there was indeed a religious event held for the occasion, and that the TAAG just missed it.

The Extremist Ritual
In KateModern it is revealed that there is a more archaic ceremony done by slaughtering the trait positive girl and drinking her blood. The rituals involved are vaguely mentioned. Kate was captured by Terrence and in the struggle he killed her, to prevent her death from going to waste he performed the extremist ceremony. But shortly learned thereafter that it had no actual effect.

Influences
Given the religious undertones of the series, it is not unlikely that the idea for the Ceremony was in part inspired by real-life religious rituals. Some rituals and beliefs that might have influenced the ideas for the Ceremony are

Body of Light
According to Crowley, the is not only used for astral travel--but in fact is the storehouse of all experiences. (From Magick Without Tears, Ch. 81):

In Magick, on the contrary, one passes through the veil of the exterior world (which, as in Yoga, but in another sense, becomes "unreal" by comparison as one passes beyond) one creates a subtle body (instrument is a better term) called the body of Light; this one develops and controls; It gains new powers as one progresses, usually by means of what is called "initiation:" finally, one carries on almost one's whole life in this Body of Light, and achieves in its own way the mastery of the Universe.

Gnostic Mass
Aleister Crowley wrote The Gnostic Mass—technically called Liber XV or "Book 15"—in 1913 while traveling in Moscow. In many ways it is similar in structure to the Mass of the Roman Catholic Church. However, the comparison ends there, as the Gnostic Mass is a celebration of the principles of Thelema. The Ceremony calls for five officers: a priest, a priestess, a deacon, and two acolytes, called “children”. The end of the ritual culminates in the consummation of the Eucharist, which is a glass of wine and the host, called a Cake of Light, after which the congregate proclaims “There is no part of me that is not of the gods!”

It is the central rite of Ordo Templi Orientis and its ecclesiastical arm, Ecclesia Gnostica Catholica.