Covenant religion

The Great Journey is a central theme for the Covenant and its theocratic government: it is their ultimate goal. This belief also stopped the Sangheili-San 'Shyuum War and united these two races. The Covenant believe that their gods, the Forerunners, used the Sacred Rings to cleanse the universe of all that is unholy, escaping a doomed existence of fighting an endless struggle against the Flood, and allowing them to transcend into gods, giving them ancient and unlimited power. The Covenant wish to activate the Halo installations to wipe out humanity and the Flood, which would provide salvation for their own doomed existences and allow them to follow the Forerunners to their mysterious destination, where they will share in their power, and take them on a path to divinity. In Covenant Religious terms the Flood are known as one of the many "obstacles" or "tests" that the Covenant must pass to begin The Great Journey.

On The Ark, during the Halo 3 campaign, several Terminals can be found left by the Forerunners, describing the activation of the Halo arrays as the Great Journey. Through such terminals on other Forerunner installations, the Covenant may have learned of the Great Journey, and used that name.

The Heretic Rebellion
"Ask the Oracle about Halo...how they would sacrifice us all for nothing!"

- Sesa 'Refumee to the Arbiter.

A few of the Covenant, called the Heretics, were aware that Halo, when activated, would destroy all sentient life in the galaxy, including themselves, thus starting the Great Journey. The Heretics were led by the Elite Sesa 'Refumee, who discovered the true purpose of Halo from 343 Guilty Spark     and came to the realization that the Great Journey and road to salvation was false. Rather than take the Covenant on a path to divinity, to activate the rings would have meant the destruction of all known forms of life in 3 radii. Filled with paranoia, the Prophets sent a group of Special Operations Elites and Grunts into their base, a Gas mine, accompanied by the new Arbiter, to silence the heretics. Sesa 'Refumee was killed by the Arbiter at the Threshold's Gas Mine before he could escape, and the heresy was put down, but still a few major heretics were alive.

The Covenant's beliefs
Whether the High Prophets really know the true effects of activating a Halo is uncertain, but the very fabric of the Covenant is built on accomplishing it, believing this will be akin to following in the footsteps of the Forerunners and propel them into the "Divine Beyond", a heavenly paradise they will share with the Forerunners. Those who are "unworthy" will be "left behind". They seem determined to activate Halo, no matter how much it takes, or costs.

The Elites were once the most devout believers beneath the Prophets. The events at Delta Halo, and their replacement by the Brutes in the Covenant, have disillusioned them to the Great Journey, and they now actively oppose those that continue to believe in it. It should also be noted that Truth told the Arbiter that the elites "never really believed in the promise of the sacred rings."

The Brutes seem blindly loyal to the Prophets, obeying every command. Now, the surviving Brutes are more than likely leaderless, with the High Prophets dead and many of their Chieftains killed in battle.

Many Grunts may not care much for religion, as they were forced into the Covenant, but those that do stayed loyal to the Prophets during the Great Schism. As a species, their opinions may be split. It's possible some Grunts may have remained loyal because they simply feared the Brutes more than the Elites, however. Most likely, they followed who ever was leading them at the time of the split.

The Jackals are believed to be more like hired mercenaries than true believers, but still remained with the Prophets during the Great Schism. Their religious beliefs are unknown, but their loyalty to the Prophets who believe unwaveringly is unquestionable.

As artificial sentient bio-mechanic organisms, Engineers were supposedly created by the Forerunners. They do not seem very religious, preferring to focus their attentions onto the technology they are responsible for. Even prior to the Great Schism, Engineers seemed to remain neutral in any fights, one even going as far as to create a gift symbolizing peace for the Humans. They most likely stayed out of the fights after the Covenant split.

Likewise, Drones and Hunters seem to feel no need for religion in their societies, but continue to follow the Prophets in their attempts to begin it. Individual beliefs may play a part in this. At least some Hunters have sided with the Elites in their opposition to it.

The Arbiter
"Here rest the vanguard of the Great Journey, every Arbiter from first to last. Each one created and consumed in times of extraordinary crisis."

- The Prophet of Truth to The Arbiter.

The currently living Arbiter, who is either leading or is an important member of the separatist movement, knows the true function of the Halo installations. At first, he had heard it from the Gravemind and the Master Chief, but it seemed as though he refused to believe the truth at the time. However, after speaking with 343 Guilty Spark, a Monitor of the lost Installation 04 who is considered by the leading Covenant caste as one of the holy oracles, he accepted it. Following this conversation he, with the help of some UNSC troops, attempted to stop the Brute Tartarus from activating Installation 05, and hence the entire remaining Halo network, but failed. In the final cutscene(s) of Halo 2 the network is described to be in a standby status, pending activation from The Ark. The Loyalists were eventually defeated, and the idea of a Great Journey lost.

Purpose
"The Great Journey waits for no one, brother. Not even you."

As the Master Chief and UNSC discovered, the Halos were not built as religious icons and representations; they are galactic weapons of mass sterilization built by the Forerunners to contain a parasitic species-devouring race known as The Flood. After exhausting every single option to destroy the Flood, the Forerunners activated the rings, allowing all sentient life in three radii of the Galactic Center to die, eventually starving and killing the Flood, and also killing themselves in the planned, organized process.

Activation
The Prophet of Truth traveled to Earth to begin the Great Journey by activating the massive structure buried beneath New Mombasa which was once thought to be the Ark. Apparently, the sole purpose of this unearthed construct is to generate a portal which leads to the true Ark, Installation 00, which lies 218 light years from the Centre of the Milky Way Galaxy.

The Prophet of Truth attempted to start The Great Journey, but was slain by the Arbiter, who stabbed Truth with an Energy Sword. The Master Chief shut down the Ark, once again stopping "The Great Journey".