I'm just copying some official answers found on the French forum for the benefit of this community:
Citation de: xilae le Juin 08, 2007, 08:29:04 AM
Baptiste Valombre Aura stipulates +1 CBT and -1 FAITH for allies in the AOE, do fighters with FAITH 0 or "-" get the bonus nonetheless?
answer: yes they get the bonus despite being unable to suffer the disadvantages associated with the Aura.