Religion is mentioned in two places in the US Constitution. One is in Article VI, Clause 3, where it states that "no religious Test shall ever be required as a Qualification to any Office or public Trust under the United States." The intent of this provision was to prohibit requiring adherence to certain religious beliefs in order to hold public office. This was important because it had been longstanding practice in England and other European countries to require officeholders to be adherents of the official church. Thus, for example, only members of the Church of England (Anglican Church) could hold office in that country--Catholics and "nonconforming" (i.e., non-Anglican) Protestants were barred from serving as government officials.
The other place where religion is mentioned is--most famously--in the First Amendment to the Constitution: "Congress shall make no law respecting an establishment of religion or prohibiting the free exercise thereof." The exact meaning of this provision has been hotly debated over the years. However, it seems to me that, at a minimum, the religion clause of the First Amendment was intended to--on the one hand- prohibit establishment of an official church or religion in the United States--and--on the other hand--prohibit the government from interfering in people's practice of their religious faith.
Some have seen these two clauses of the US Constitution--the religious tests clause and the non-establishment clause--as reflecting an intent to protect the government from religious influence. However, according to Professor Robert P. George of Princeton University in an interview with the George W. Bush Institute, this is incorrect:
The mistake some people make is to suppose that, in these provisions, the Constitution's framers and ratifiers are attempting to shield the state from religious influences or protect the state from religion. It's quite the opposite of that.
These provisions protect the rights of religious folks of all faiths and shades of belief against predation by the national government, and in some cases by the states. They don't want government interfering in, for example, the free exercise of people's religion. The Constitution understands that religion is very precious, that it is how we attempt to understand the world and relate ourselves to the ultimate source or sources of meaning and value. (for the complete interview, see here).
However, regardless of what one believes about the meaning of the Constitution's provisions on religion, it would seem from the actual text of the Constitution that it has no foundation in religious belief. After all, unlike that other foundational document of the United States, the Declaration of Independence, it contains no reference to "Nature's God" or "divine Providence." However, I would argue that despite its lack of reference to God, the Constitution does presuppose a certain view of humanity rooted in Judeo-Christian belief. That view is that human beings are naturally inclined toward doing evil. Consequently, government is needed to restrain the evil that men and women might do. As James Madison, known as the "Father of the Constitution" for his many contributions in its drafting, wrote in The Federalist: "If men were angels, no government would be necessary" (although, according to the Bible, even angels can commit evil!). However, government itself can create opportunities for doing evil, especially if political power is concentrated in the hands of one person or a small number of people--hence the whole system of checks and balances and separation of powers in the Constitution.
Contrast this with the political system of an explicitly atheistic government like that of the People's Republic of China, in which political power is in the hands of a relatively small number of people--the Communist Party--who are supposedly virtuous enough not to abuse that power.
In sum, the US Constitution protects the rights of individuals to hold whatever religious beliefs they will and to practice those beliefs, while being based on a view of human nature that derives from religious beliefs. And that, I would contend, is something worth celebrating!
Image of Independence Hall, where the US Constitution was signed, from Wikimedia Commons