How dumb a political idea was it for Cercei to allow the faith militant to be reborn, considering that she got the debt and acknowledgement of Tommen in return? I get that it was a bad idea but no one should’ve expected to be as bad as it was, right?

Given that the last time the Faith Militant existed, they revolted against the Iron Throne and came pretty damn close to toppling the monarchy back when the Targaryens still had dragons, I think it’s entirely forseeable that re-instituting it in the midst of a religiously-inspired populist movement is going to be very bad for the monarchy. 

Which is why literally everyone around her is telling her it’s a terrible idea.