Originally Posted by Brother Lucian
Look at Celestine. Pretty much a daemon of the Emperor inhabiting a flesh puppet, with how she calls down the Emperors will to smite the enemy in a beam of light from on high.
There are any number of things that could explain Celestine's abilities. Given the Emperor's aversion to being worshiped as a god, I am inclined to believe her abilities and acts of faith are the result of the heavy presence of faith her vicinity on the battlefield. When she's around, human soldiers of the Imperium believe their victory is assured, they believe an avatar of the Emperor's will is with them and the ripples this could cause in the warp may explain events in her presence.
There is also the possibility she is a latent psyker or some kind of psychic mutant and her gifts have simply been misinterpreted.