Before answering this question, it is crucial to know what it means. Several possibilities exist.
WERE THE ORIGINAL SETTLERS CHRISTIANS, AT LEAST NOMINALLY?
Of course they were. Since the colonists were overwhelmingly from Europe, their religion, if they adhered to one, was Christian. Most of the colonies had some from of religious establishment, whereby one or more Christian denominations enjoyed a privileged position, including taxpayer support. Those who belonged to other denominations, or to none, were typically at a disadvantage, sometimes severely so. Establishment led, in varying degrees, to persecution and injustice.
DO MOST AMERICANS PROFESS TO BE CHRISTIANS?
More Americans profess Christianity than any other religion. At one point, the number adhering to other religions, or to none, was small. Today, it is substantial and growing, with only about seventy-six percent claiming to be Christians and considerably fewer affiliated with a Christian church—at least a third of American adults seldom or never attend any religious service. If the question is about demographics, we can say that the United States is a majority Christian nation, at least for now, if we count unaffiliated people.
IS AMERICAN CULTURE CHRISTIAN, BY HERITAGE AND PRACTICE?
From the first settlers to the present, though in diminishing proportions, most Americans have received their ethical and moral instruction in Christian settings or from socializing agents (usually parents and teachers) who considered themselves to be Christian. Many of these instructions, such as prohibition of murder, theft, robbery, rape, adultery, and lying, are widespread among human cultures and not distinctively Christian. No one would argue that any nation prohibiting such actions is therefore a Christian nation. The distinctively religious precepts of the (Hebraic) Ten Commandments--no other God, no graven images, taking the name of God in vain (if interpreted as blasphemy rather than perjury), keeping the Sabbath--are not part of our laws and few seem interested in making them part.
If adherence to the teachings of Jesus, or of the Bible as a whole, is the criterion, the claim that the United States is a Christian nation is equally difficult to sustain. Displacing and exterminating most of the country's indigenous native population and sanctioning slavery in the Constitution are, to choose only two, hardly shining examples of this allegedly formative ethos. As those who argue most strongly that America is a Christian nation readily admit, the dominant popular culture offers modest evidence of its Judeo-Christian origins. Similarly, one may support public policy that favors the rich over the poor or that places its trust in overwhelming military might, but tracing the distinctively Christian origins of those policies is a challenge.
Christian culture does, of course, exert a strong influence. Many businesses close on Sunday and Christmas is a national holiday. People often lead public prayers "in Jesus' name," even though non-Christians are present. It is easily possible to think of other examples.
DID THE FRAMERS OF THE CONSTITUTION INTEND TO ESTABLISH A CHRISTIAN STATE?
Those who contend that the United States is a Christian nation typically maintain that the Framers of the Constitution assumed as much, though they did not state it explicitly in the Constitution, and certainly never intended to erect a wall of separation between church and state. The implication of this view is that the United States, as a political entity, was intended to be a Christian state and that Christianity was intended to have and should continue to have a privileged status within the United States, and that adherents of other religions, or of no religion, are not entitled to the same considerations.
In my experience, most people who contend that America is a Christian nation subscribe to this view, either explicitly or implicitly. It is this position that I want to oppose.