(F)undamentalist religious doctrines and autocratic and dictatorial rulers will reject the ideas of public reason and deliberative democracy. They will say that democracy leads to a culture contrary to their religion, or denies the values that only autocratic or dictatorial rule can secure.  They assert that the religiously true, or the philosophically true, overrides the politically reasonable. We simply say that such a doctrine is politically unreasonable... Unreasonable doctrines are a threat to democratic institutions, since it is impossible  for them to abide by a constitutional regime except as a modus vivendi. Their existence sets a limit to the aim of fully realizing a reasonable democratic society with its ideal of public reason and the idea of legitimate law. This fact is not a defect or failure of the idea of public reason, but rather it indicates that there are limits to what public reason can accomplish. It does not diminish the great value and importance of attempting to realize that ideal to the fullest extent possible.
--John Rawls, "The Idea of Public Reason Revisited," Chicago Law Review (1997), 64 (3): 765-807.