Truth of Sigmar

The Unifiers, who believe Sigmar is the Emperor of the Gods, are not generally extremists. Members of the Truth of Sigmar are; they believe Sigmar is the only true God remaining. They say that after Ulric crowned Sigmar as a God, the Gods went to war against the Ruinous Powers, and all save Sigmar were killed. The Dark Gods continued to support the priests of other faiths, while slowly perverting them into following dreadful rites. Sigmar, although he stood alone, was able to beat the Dark Powers back into the Wastes, but even for Sigmar, the fight is a difficult one.

Members of the Truth of Sigmar thus believe anyone who pays any respect to another deity is, in truth, a Chaos cultist. Such worship helps the Ruinous Powers, and most take the Storm of Chaos as a sign that Sigmar is finding it harder to hold back the forces of darkness. This is, of course, also a call to action: eliminate the worshippers of all other deities.

The zealots of this group stay as secret as possible, because they do not want to draw the attention of their enemies. They focus their attacks on temples and priests of other deities, aiming to destroy without mercy. They do include open worshippers of the Ruinous Powers among their enemies, and have destroyed genuine chaotic cults. They have, however, killed far more relatively innocent priests of other deities, and damaged or destroyed a large number of temples and shrines.

The Order of the Cleansing Flame is aware of the existence of this group, and hunts it down mercilessly. Not only are they heretics, but by fighting against the enemies of Chaos, they weaken the Cult of Sigmar in the long battle against corruption. A few priests are members of the group, but no one in a true position of authority is believed to be affiliated with them.

Source
pg. 108
 * Warhammer Fantasy RPG 2nd ED -- Tome of Salvation