The entire Covenant's religion is based around the Forerunners, where did this originate? To answer this question, we have to go all the way back before the Halo Arrays were fired approximately 100,000 years ago.
Before all sentient life in the galaxy was destroyed, the San'Shyuum race were much different than what we see today. The San'Shyuum of the time were a handsome and intelligent race, more concerned with sensuous beauty, eternal sexuality, and youth. Their intelligence led them to create highly advanced technology, even rivalling the Forerunners. The San'Shyuum were active in trading goods with humans, trading not only food, but also technology and even live animals. One of these animals was the Pheru, which were later the first infected by the Flood.
This infection caused the humans and San'Shyuum to kill any creature that showed symptoms, but it was not enough. The ensuing Human-Flood war was misinterpreted by the Forerunners as an aggressive form of colonisation, while the humans were fleeing the Flood; this pushed the humans in a second concurrent war, the Human-Forerunner war. The San'Shyuum decided to join the humans in their fight against the Forerunners, and together they formed an incredibly strong force. The Human-San'Shyuum alliance fought as well as they could, but they were fighting a losing war. The battle on two fronts was too much, and soon the humans and San'Shyuum were defeated. As punishment, the humans were 'deevolved' thanks to the Librarian while the San'Shyuum were put into quarantine on their home-planet Janjur Qom. They were put under the supervision of the Confirmer aboard the Deep Reverence, the Confirmer had this responsibility for over twelve centuries without being relieved of his duty. Due to the greatly deteriorated mental and physical well-being of the Confirmer, he had grown lax in his observations. This gave the San'Shyuum room to smuggle in weaponry to prepare for an uprising.
During the Forerunner-Flood war, The Librarian visited Janjur Qom to secure specimens of San'Shyuum for preservation from the Halo Array. The San'Shyuum elders correctly guessed that a catastrophe was imminent, and launched a short-lived uprising against their Forerunner overseers. All San'Shyuum on the planet were subsequently killed when a Halo was fired on a low power setting nearby.
Rising from the ashes
The firing of the Halo Array caused the destruction of the San'Shyuum and their culture. It was not until later that they would be reseeded from Installation 00. During the exploration of their 'new' home-planet, they discovered much of the Forerunner technology which was left behind. Believing the items were left behind by the gods the San'Shyuum based their religion around it. The main piece of Forerunner technology was a Forerunner dreadnought, and their religion was very strict that nobody should be allowed to even touch it.
The Dreadnought was the central point of all worship by the San'Shyuum, while they were building up their technology. But when the San'Shyuum technology became advanced to enough to enter space-flight, a faction arose that wanted to use the Dreadnought. This group, called the Reformists was greatly opposed by the Stoics, who saw this act as a desecration of the gods. This sparked the extraordinary bloody San'Shyuum Civil War, which lasted for a century. For most of the war, the Stoics were able to but down the Reformist's attacks, but where ultimately overwhelmed by the Forerunner weapons the Reformists were using. The war finally ended when a group of Reformists entered the Dreadnought and flew away, taking a large chunk of the planet with them. The Forerunner dreadnought together with the large land-mass would later be known as High Charity. The land-mass would be formed after the lonely ship that had orbited their homeworld for so many centuries, the Deep Reverence.
Even though the Reformers were victorious and had left the Stoics (who sent out communication messages threatening that they would be damned for their digressions to the gods) behind, they realized that they were indeed ruined by the fact that there were only about a thousand that had joined their cause to control the Dreadnought. With an extremely small gene pool they would need to create strict controls around mating to prevent potentially disastrous inbreeding.
Forming the Covenant
During their interstellar journey, the San'Shyuum came across the planet of Ulgethon in the year 938 BCE. On this planet was a large Sangheili presence, who, like the Stoics, worshipped the Forerunner's relics; this ideological difference resulted in the Sangheili-San'Shyuum war which would last for 86 years. At first, the Sangheili had the upper hand, with superior fighting skill and strategic planning. It was not until the San'Shyuum started to use their technology more, that the war turned in their favour. The Sangheili did have large fleets of ships, but these were no match for the hit-and-run strategy the San'Shyuum employed with the Dreadnought.
The fighting took place on several planets, with the Sangheili defending their homeworld Sanghelios the fiercest. During these battles, the Sangheili and San'Shyuum became more and more familiar with each other ideologies, most notably the San'Shyuums believe about the 'Great Journey'. The San'Shyuum believed that the Great Journey would begin after the firing of a Halo Array, and transcend them, and other believers, into god-like creature.
Near the end, victory for the San'Shyuum and the end of the Sangheili race seemed inevitable. It was in the year 876 BCE that the Sangheili warrior priests finally, and grudgingly began to use their Forerunner artefacts to improve their warship, weapon and armour designs. This frightened the San'Shyuum to a great extent, fearing that there may have been other races with the same martial capabilities. If this was the case, the San'Shyuum would have to repeat this same war over and over. In order to prevent this prospect, and to prevent the extinction of the Sangheili race, the San'Shyuum and Sangheili leaders came together to form a peace.
So in 852 BCE, the San'Shyuum and Sangheili wrote and signed the Writ of Union, and the Covenant Empire was formed. The full version of the Writ of Union has not been released, but it starts as follows:
- “So full of hate were our eyes
That none of us could see
Our war would yield countless dead
But never victory
So let us cast arms aside
And like discard our wrath
Thou, in faith, will keep us safe
Whilst we find the path”— The beginning of the Writ of Union
This piece clearly indicates that both parties decided that the way to the 'Great Journey' was together. The San'Shyuum took up the role as religious leaders and scientists, while the Sangheili would serve as the San'Shyuum's protectors and warriors, having total control over all security matters.