.... America is not a Christian country and has never been, for historical, theological and philosophic reasons.
First, the Constitution was designed for religious diversity because the Founders were religiously diverse. The 18th century was a time not of quiet piety but of religious controversy. It was a high tide of American Unitarianism, a direct challenge to Christian orthodoxy. Thomas Jefferson's deism flirted with atheism -- a God so distant that He didn't even require his own existence. As journalist Jon Meacham points out, the Founders were less orthodox than the generation that preceded them, as well as the one that followed them. Their commitment to disestablishment, in some cases, accommodated their own heterodoxy.
Second, American religious communities were often strong supporters of disestablishment. Dissenting Protestants had a long history of resentment for the established English church. Others -- Catholics and Quakers -- were minorities suspicious of majority religious rule. Christians generally saw state intrusion as a threat to their theological integrity and worldly power as a diversion from their mission. They supported disestablishment for the sake of the church. And their political independence contributed to their religious vitality.