Literally «an act of faith» but in usage its meaning is exclusively a sinister act performed in the name of a greater good. May refer specifically to the torture and burnings of the Spanish Inquisition.
I wrote this down a while back but I'm pretty sure that I found it in de Tocqueville's Democracy in America.
This takes the prize for most loaded phrase. Many phrases are used almost exclusvely in the ironic sense, but few have made it into the dictionary (Chambers' and Oxford American were the ones I checked) with so specifically a negative connotation when the literal meaning is at worst neutral and at best virtuous. The Constitution has the full faith and credit clause, people are asked to take information on faith and courageous acts have been described as acts of faith, but an English speaker using auto-da-fé is certainly speaking of evil done in the name of good, and most likely referring to the Inquisition.