Auctoritas (Nobilis)

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The Auctoritas is the highest expression of Spirit, of a Power's selfhood[sic]. It is the Space within about five feet of the Power, in every direction: it is the tie between the Power and the tree Yggdrasil, where all worlds hang. This mystic shield turns away the magics of other Powers, Imperators and even Excrucians. When two Powers each enter the Auctoritas of the other, there is an uncomfortable feeling, like a grating sandstone sound in the mind and in the soul.

Auctoritas in a Nutshell
Auctoritas works like a spiritual armor that protects Characters against direct effect Miracles (though this can be cleverly worked around if the caster uses the Miracle to create desirable effects on the target without the Miracle being foiled by the target's Auctoritas, such as create a Anvil above the Target and drop it, figuratively speaking off course, even though this is allowed and encouraged).

As accordingly to the official rules (or lack of any mention of the subject) the only way to increase the Auctoritas is to directly increase the Spirit Attribute value. Additional House Rules can be created to allow Character Points to be spent in order to increase both the Auctoritas value, it's range and add additional properties (such as +2 Auctoritas against Realm Miracles or -2 Auctoritas against Aspect Miracles, for example).