Christianity is no longer "the air we breathe" in America. This is not necessarily bad. American "Christianity" was not necessarily representative of authentic Jesus-following. And, real Christianity has flourished and been a revolutionary force when marginalized and persecuted.
In his book The Naked Anabaptist: The Bare Essentials of a Radical Faith, Stuart Murray gives the following marks of our post-Christian culture.
Do not be discouraged by this! "In today’s post-Christian culture, Christians would do well to see these changes as opportunities to re-embrace our subversive roots and follow Jesus from the margins. The natural outcome could be a witness characterized by the fruit of the Spirit, enemy love, and justice for the poor. Perhaps Christian credibility will be revived afresh." (Kurt Willems)