The separation of church and state is necessary partly because if religion is good then the state shouldn't interfere with the religious vision or with the religious prophet. There must be a realm of truth beyond political competence, that's why there must be a separation of churches, but if religion is bad and a bad religion is one that gives an ultimate sanctity to some particular cause. Then religion mustn't interfere with the state - so one of the basic Democratic principles as we know it in America is the separation of church and state.
Reinhold NiebuhrOur dreams of bringing the whole of human history under the control of the human will are ironically refuted by the fact that no group of idealists can easily move the pattern of history toward the desired goal of peace and justice. The recalcitrant forces in the historical drama have a power and persistence beyond our reckoning.
Reinhold Niebuhr