Recently I was reading an article from the George Barna group, in which the contention was made that Christianity was no longer America's "default" religion. Much as it was usually assumed that if you were of a particular nationality, you were of a particular religion (ie, Irish were Catholics, Arabs were Muslims, residents of Lakeland, FL were Methodists), it was typically assumed that Americans were some form of Christian.
1Co 9:20 To the Jews I became as a Jew, in order to win Jews. To those under the law I became as one under the law (though not being myself under the law) that I might win those under the law.
1Co 9:21 To those outside the law I became as one outside the law (not being outside the law of God but under the law of Christ) that I might win those outside the law.
1Co 9:23 I do it all for the sake of the gospel, that I may share with them in its blessings.