During the Ages of Reconciliation the San'Shyuum race shared the information they had learned about the Forerunners with the Sangheili. The Sangheili and the San'Shyuum made a truce in order to unite and form the Covenant, with the Sangheili being promised salvation in exchange for their services as the military force and bodyguard of the Prophets. Presumably, this is when the Covenant Writ of Union was created, although it may have been later on when more races joined in servitude to the Prophets.
Progression of the AgesEdit
Leader, or Chieftain: Reconciliation.
Congregation, or Pack: The Third! Humility and brotherhood!