Manifest Auctoritas: Limit of Spirit
Like the robes and Death Mask of Thanatos in Incarnations of Immortality, or Morpheus's helmet in Sandman, the Noble's Auctoritas is invested in a tool of the office rather than being innate (or at least the Noble believes that it is). The Power of Fiction might have a science-fictional shield generator, while the Power of Passions could wear a dress made of lovers' tears.
This object is invulnerable to harm (except perhaps by Abhorrent Weapons). However, it can be taken away or lost by some means, which means that miracles no longer need penetration to affect the Noble. It also likely ends the effects of the Rite of Holy Fire. Needless to say, this is a dangerous situation, both for the Noble and for their Imperator (akin to leaving a gate of a fort open and unguarded during wartime), and Ymera will be extremely insistent that the Noble rectify the situation, should it learn of the situation.
Some Nobles may only lose their manifest auctoritas by some particular means, such as being presented with some specific offering or symbol, being tricked into giving it up, or being forced to surrender it to anyone that thinks to request it and states their goal for taking it honestly. No other method of removing it, including physical force, will succeed. (GMs may, of course, veto this limit if it seems abusive or unlikely to ever come into play.)