"Do you, Garviel Loken, accept your role in this? Do you promise to lead your men into the zone of war, and conduct them to glory, no matter the ferocity or ingenuity of the foe? Do you swear to crush the insurgents of Sixty-Three Nineteen, despite all they might throw at you? Do you pledge to do honour to the XVI Legion and the Emperor?"
-"On this matter, and by this weapon, I swear."
An Oath of Moment is an ancient tradition that harkens back to the time of the heroic Space Marine Legions during the Great Crusade and Horus Heresy era in the late 30th and early 31st Millennia. These were solemn promises, often invoked by several witnesses, usually fellow Battle-Brothers, regarding an individual's future actions in regards to a sacred vow or pledge to be carried out no matter the cost. An Oath of Moment was often sworn before going into battle or setting out on a mission and was taken as a reaffirmation of the sacred oaths made to the Emperor of Mankind and a Space Marine's individual Primarch or Legion. A typical Oath of Moment often required at least two persons -- the individual Astartes swearing the oath and a Battle-Brother(s) to offer it and witness it.
The essence of such oaths was an invocation to be a guarantor of the oath-taker's honesty and integrity in the matter under question. By implication, this invokes divine displeasure from an Astartes' Primarch or the Emperor Himself if the oath-taker failed in their sworn duties. Therefore, the import of such a significant pledge implied greater care than usual in the act of the performance of an individual's duty. These pledges usually had an oath-taker swearing them over a weapon. Often times these oaths were written on parchment and affixed to the oath-taker's Power Armour before he departed on a mission.
A Space Marine squad often swore an Oath of Moment collectively, but an isolated Space Marine could swear one simply by declaring it aloud during the heat of battle on an uncaring battlefield or even in his own mind to steel his resolve, with the Emperor as his witness. Some Legions kept these oath parchments as a record of tasks achieved while others more than likely disposed of them ceremoniously or discarded them after they had achieved the sworn promise and were thus no longer relevant. The Space Wolves for example, often eschewed the use of such oaths, scorning the idea of writing them down. As one Long Fang at the time of the Fall of Prospero pointed out, "That's what memories are for."