It is common to hear the claim today that the United States was founded as a Christian nation, but in reality the Constitution was a document of civil law with little reference to religion. In those places that religion is mentioned, the obvious motive is to make sure that religion did not come to control government.
In the ratification process, an amendment was added to make sure that the government would not restrict freedom of religion.
The nature of the Constitution as a civil law with protections from religious influence and the nature of the amendment preventing government from restricting freedom of religion came to be known as “separation of church and state.”
It is not surprising that the framers of the Constitution made these provisions. Among the framers there were a couple of Roman Catholics, several Deists, a smattering of atheists, and representatives of half a dozen protestant denominations. They were all acutely aware of the control that the church exercised over governments in Europe, and they were just as aware of the religious persecution of minority religions, free-thinkers, and dissenters that was common in countries with established religions. Separation of church and state underpins religious freedom and the civil right to freedom of speech, neither of which can exist without the dividing line. This separation with safeguards from both sides is one of the most ingenious ideas embedded in the Constitution.
The discussion below is from a page devoted to the Constitution, The U.S. Constitution Online. The entire page is well worth visiting, and this section deals specifically with how the Constitution referenced religion:
Religion in the original Constitution
Religion makes only one direct and obvious appearance in the original Constitution that seems to point to a desire for some degree of religious freedom. That appearance is in Article 6, at the end of the third clause:
[N]o religious Test shall ever be required as a Qualification to any Office or public Trust under the United States.
This statement is simple and straight-forward, and applies to all offices in the entire United States, both state and federal. The clause simply means that no public position can be required to be held by any one of any religious denomination. It would be unconstitutional for there to be a requirement that the President by Lutheran, or even for the mayor of a small town to be Christian. Likewise, it would be unconstitutional for a law to forbid a Jew or Muslim from holding any office in any governmental jurisdiction in the United States. (This having been said, it should be noted that several state constitutions do have a religious test — specifically, they deny office to anyone unwilling to acknowledge God or a Supreme Being.)
In the debates of the Constitutional Convention, religion did not get a lot of sound bites. It should be noted that without exception, the Framers were Christian or, at the very least, deists (generally, deists believe in a single god who set the universe on its course and then stepped back to watch; some deists believe their deity is the same God of Judeo-Christian tradition, some do not). There were no Jews or Muslims, no Hindus or atheists, and only two Roman Catholics. There were members of more than a half-dozen sects of the Protestant side of Christianity, though. Disagreements about style and method of worship between them were nearly as vast and incongruous as any seen today between, say, Jews and Muslims, such that the Framers wanted to ensure that no one sect could ever seize control of a government and start a theocracy.
James Madison, when speaking of the method and manner of the election of the members of the Congress, noted that even “Religion itself may become a motive to persecution and oppression,” telegraphing his own desire for no religious test for government service. He had been a prime mover in the efforts of some Virginia lawmakers to ensure that no preference be given to any religion in that state, and that a proposed tax to aid religious efforts be defeated. Madison and one of the Pinkney cousins moved, in the waning days of the Convention, that the Congress be permitted the power to establish a university, with the express stipulation that “no preferences or distinctions should be allowed on account of Religion.” The motion was turned down on a six to four vote, but it was another illustration of his desire to extend no preference to any religious sect.
There is one other direct bow to religion in the original Constitution, and it is a bit obtuse. The Presidential Oath of Office is codified in the Constitution in this way:
I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States.
Again, the reference might be obtuse, but it is the inclusion of language in the oath that allows an incoming President to swear or affirm the oath. This alternate text has been described both as a way of accommodating those religious persons for whom “swearing” was forbidden, and as a way for the unreligious to take the oath with the same force of personal responsibility that swearing would have for a religious person. Either way, the alternate text attempts to make the oath all-inclusive and religion-neutral.
Finally, the Constitution refers to the year that the Convention created the document as “the Year of our Lord one thousand seven hundred and Eighty seven.” Some have argued that the use of the term “Lord” in this way is indicative of something, but it is indicative of nothing more than a standard way of referring to years in that time period.