Gesture of Sincerity
When she’s trying to convince someone of something, the Avatar puts her hand to her heart, puts her hand on a stack of Bibles, holds up her right hand with her pinkie and thumb crossed in the palm, or performs some other gesture of sincerity. (Her player also spends a point of Divinity.) When she does so, her would-be mark accepts that the Avatar is telling the truth about the subject at hand, no questions asked, for the rest of the story.
Only solid, incontrovertible proof showing that what the Avatar said was undeniably false will convince the mark he’s been fooled. Even then, whoever is showing him the proof will have to convince him that the proof is genuine and not some clever forgery.
A Avatar with this Flair doesn’t have to lie when she uses it. She can also use it to convince a recalcitrant skeptic of the actual truth. If the Avatar does so, no mortal force can convince that person that the Avatar was lying (not even professionally faked “proof” to the contrary).