Writ of Union

The Writ of Union is an oath of service to the Covenant, written by the Prophets and perhaps Elites who created the Covenant, apparently the Covenant High Council of Concordance who issued this. In the current Covenant tradition, when an Elite becomes a warrior, they recite the oath. This "Writ" was apparently authored and first documented in the first Age of Reconciliation.

The Oath
The full content is unknown, but here are some excerpts:

The Arbiter: So full of hate were our eyes that none of us could see.

Our war would yield countless dead, but never victory.

Prophet of Truth: So let us cast arms aside, and like discard our wrath.

Thou, in faith, will keep us safe, whilst we find the path.

On the Halo 2 level The Arbiter, in the opening cutscene, SpecOps Elites recite this:

SpecOps Commander: "When we joined the Covenant, we took an oath!"

SpecOps Elites: "According to our station! All without exception!"

SpecOps Commander: "On the blood of our fathers... on the blood of our sons, we swore to uphold the Covenant!"

SpecOps Elites: "Even to our dying breath!"

SpecOps Commander: "Those who would break this oath are heretics... worthy of neither pity, nor mercy!"

SpecOps Elites: "We shall grind them into dust... Scrape them as excrement from our boots."

SpecOps Commander: "And continue our march to glorious salvation!"

On and on shall old war go,

Without respite my blood will flow

O’er your eyes ‘til they cannot see

The impossibility of victory.

Trivia

 * Covenant Deacons, such as the Unggoy Dadab, were required to memorize the entire Writ of Union.
 * During gameplay an extra line is added to the oath by the Special Operations Elites after "Even to our dying breath" in the subtitles, however remains unspoken "and scrape them as excrement from our boots".