John Adams on Religion and the Constitution
Found in The Works of John Adams, 10 vols.
This quote is from John Adams (1734-1826), a signatory of the Declaration of Independence, one of the framers of the Constitution, and the second president of the United States of America. This quote, and especially its famous last line, expresses a sentiment widely held among the statesmen of the Founding Generation that no matter how well a constitution is constructed, It will not insure freedom and prosperity unless it is supported by a moral, virtuous population.
While our country remains untainted with the principles and manners which are now producing desolation in so many parts of the world; while she continues sincere, and incapable of insidious and impious policy, we shall have the strongest reason to rejoice in the local destination assigned us by Providence. But should the people of America once become capable of that deep simulation towards one another, and towards foreign nations, which assumes the language of justice and moderation while it is practising iniquity and extravagance, and displays  in the most captivating manner the charming pictures of candor, frankness, and sincerity, while it is rioting in rapine and insolence, this country will be the most miserable habitation in the world; because we have no government armed with power capable of contending with human passions unbridled by morality and religion. Avarice, ambition, revenge, or gallantry, would break the strongest cords of our Constitution as a whale goes through a net. Our Constitution was made only for a moral and religious people. It is wholly inadequate to the government of any other. (FROM TO THE OFFICERS OF THE FIRST BRIGADE OF THE THIRD DIVISION OF THE MILITIA OF MASSACHUSETTS, 11 October, 1798)
Adams, and many others, believed that in a society riddled with vices, the various mechanisms created by the Constitution would not be able to function properly, with the result that the democratic republican order would be eventually supplanted by despotism.
It is worth noting, however, that while Adams occasionally made positive references to Christianity, he was not a strong advocate of any particular religion (at least in his public statements) or even a specific moral code, except insofar as it condemned a (rather short) list of obnoxious vices. Indeed, Adams was firmly against the official establishment of any religion and was a champion of religious freedom. As Deists, Adams and his like-minded peers believed that any moral system had to be grounded in a belief in God, but the specific aspects of that belief, or how they manifested themselves in a particular religion, were of little if any interest to them.