Consider the time in which the original Inquisition existed. The First Blight had devastated the world, tearing down the mighty Tevinter Imperium and leaving nothing in its place. Just as there were several cults of Andraste vying to become the true inheritor of the prophet's faith, there was more than one group claiming to represent the one true path to deliver the world from chaos. The Order of Fiery Promise was one of these, a band of men and women who decreed that not only was the end of the world nigh, it was necessary.
Thedas must be cleansed with fire and reborn as a paradise. This they solemnly promised; they devoted themselves utterly to seeing this come about. Whether they ever drew closer to their goal is unknown.
The Inquisition eventually crushed these "Promisers" during a battle only recorded as the "Cleansing of Churneau." The cult reappeared in the early Divine Age, claiming to take up the Inquisiton's mantle after it had transformed into the Seekers of Truth, leading to Chantry hunts that did not see the Promisers eliminated again until the Exalted Age.
—From In Pursuit of Knowledge: The Travels of a Chantry Scholar by Brother Genitivi