Long ago, Remnant was created by Oum, God. He created all life, including the Grimm, to populate His world. He knew what kind of creatures the Grimm were, for they were meant to be the way they were, and so in order to protect the humans, His favorite of all of His creations, He created the necessary supplies for humanity to fight against the Grimm.
But the humans never figured it out; they fell before the waves of Grimm, helpless and childlike, and, in turn, doomed themselves with the negativity they constantly bore for negativity attracted the Grimm like flies to honey. And so Oum created the Divine One, a devout follower of Him with the ability to use His language, the language of creation, to do whatever was needed.
The Divine One helped to save all of humanity by leading them to Dust - the necessary weapon in order to vanquish the Grimm. The Divine One protected humanity as they mastered the use of this new weapon, this new tool used in order to save their lives, and when the job was finished and over with, the Divine One went into the wilderness and died.
Few history books date back to this time, but those that do all document the Divine One's last statement: "I will be reborn. I will be reborn sometime in the future, though I do not know when - when the fate of humanity, once again, hangs in the balance."
Legend has it that when the Divine One is reborn, the reincarnation will have to make a choice, one that will decide the fate of humanity.
Well, I guess we'll just have to wait and see what I choose, huh?