Mantle

"The Mantle. You still hold to that [belief] after all that has happened? After this thing has consumed a million worlds? Can't you see? Belief in the Mantle sealed our doom! Weakened our [protectorates], bred dependence and sloth. Our [so-called Guardianship] has stripped those we would keep safe of any capacity for self-defense! Were we such noble [Guardians] when we drew our line and abandoned billions to the parasite?"

- The Librarian

The Mantle was a Forerunner belief system in which the Forerunners upheld they were required to protect and promote life. Bridging the social strata between an interstellar Marshall Plan and a religious, but benevolent, stewardship, the Forerunners took responsibility for the protection and cultivation of the species and planetary systems within their domain. The Mantle was based around the concept of Living Time; "the joy of life’s interaction with the Cosmos". This was also the origin of all of the complex rules involved in the Mantle.

History
Forerunner mythology held that the Mantle was passed down to them by an even earlier and more advanced race, the Precursors; to what extent this was true, or even whether or not it was a common belief, remains unknown. The Forerunners believed themselves to be only a passing stage in the universe's Living Time, and that better civilizations would come after them. This is also why they chose to use the name "Forerunner" to refer to themselves. Regardless of its origin, the Forerunners seem to have handed their Mantle down to humanity; it may play a significant part in humanity's Reclaimer status.

Some Forerunners doubted the purpose and validity of the Mantle, dismissing it as superstition and fairy tales. The Librarian, in particular, had such doubts, and even went so far as to hold it responsible for the downfall of the Forerunners at the hands of the Flood. The then-rampant AI Mendicant Bias believed that the Forerunners used the Mantle to obstruct any change, even that which might eventually be beneficial.

Their central government was entirely dictated by the Mantle, as were the religious beliefs of individual Forerunners. However, inspiring peace throughout their domain by minimizing their military capacity and disarming their weapons caches proved to be a costly approach for the Forerunners when the new alien threat emerged.

Practices
Despite its ultimate ideal to preserve all life, questionable practices were also committed in the name of the Mantle. As the Forerunners viewed themselves to be above all other species, their Warrior-Servants would not hesitate to wipe out those who stood against them, believing that to defy the Forerunners was to show contempt on the Mantle itself. Their perceived superiority also caused the Forerunners to sequester themselves from other species; allegedly, this was done to protect and preserve all species, including themselves. In addition, the Forerunners considered it heretical for any other species to claim to be true inheritors of the Mantle.

Rather than allowing every species to rise on their own merits, the Forerunners, following their Mantle, gave them no need to do so, serving as protectors against any threats that might have forced technological or cultural progression. An unforeseen side-effect was that only the Forerunners were able to hold their own against the Flood (at least once they rearmed), while their charges were unable to defend themselves; the Forerunners had unintentionally left the galaxy vulnerable to Flood infection.

The Mantle's religious texts were divided into chapters known as "Numbers". Notable Forerunners, such as the Didact, would apparently contribute to the Mantle and have their own "Numbers" which contained short aphorisms or phrases known as "Permutations". The Twelve Laws of Making and Moving were a central religious text of the Mantle often recited in prayers. A Master of the Mantle was an individual, likely a religious official, whose duties included securing a Forerunner's physical remains into a Durance after an immolation ceremony.