This problem became topical for me with the recent observation of a friend, who noted that Christianity is in severe decline in contemporary society, and that contemporary society is not the least bit affected by this trend.
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As government retreats, religion generally expands. Is it any coincidence that the religious revival of the 80s and 90s parallels the rising of conservatism?
Religion often performs many functions that governemnt often tyrnanically assumes, thus governments have historically been very wary of religion because religion can foster independent thought and form strong political movements. This is why most communist countries banned all religions and other tyrants (middle eastern) make sure they control religion or merge the two together (Iran).
So, as long as we keep expanding freedom, I think we have nothing to worry about regarding Christianity expanding.
Where has government retreated? The growth of power and size of the government has coincided with growth in conservatism and religious revivalism. W has increased government spending, control, power, influence, etc., more than anyone since Wilson.