Sir William Blackstone
[W]e have no government armed with power capable of contending with human passions unbridled by morality and religion....Our constitution was made only for a moral and religious people. It is wholly inadequate to the government of any other. (Source: John Adams, The Works of John Adams, Second President of the United States, Charles Francis Adams, editor (Boston: Little, Brown, and Co. 1854), Vol. IX, p. 229, October 11, 1798.)
Without morals a republic cannot subsist any length of time; they therefore who are decrying the Christian religion, whose morality is so sublime & pure, [and] which denounces against the wicked eternal misery, and [which] insured to the good eternal happiness, are undermining the solid foundation of morals, the best security for the duration of free governments. (Source: Bernard C. Steiner, The Life and Correspondence of James McHenry (Cleveland: The Burrows Brothers, 1907), p. 475. In a letter from Charles Carroll to James McHenry of November 4, 1800.)
[O]nly a virtuous people are capable of freedom. As nations become corrupt and vicious, they have more need of masters. Source: Benjamin Franklin, The Writings of Benjamin Franklin, Jared Sparks, editor (Boston: Tappan, Whittemore and Mason, 1840), Vol. X, p. 297, April 17, 1787.
Of all the dispositions and habits which lead to political prosperity, religion and morality are indispensable supports. In vain would that man claim the tribute of patriotism who should labor to subvert these great pillars of human happiness, these firmest props of the duties of man and citizens. The mere politician, equally with the pious man, ought to respect and to cherish them. A volume could not trace all their connexions with private and public felicity. Let it simply be asked, Where is the security for property, for reputation, for life, if the sense of religious obligation desert the oaths, which are the instruments of investigation in Courts of Justice?
And let us with caution indulge the supposition that morality can be maintained without religion. Whatever may be conceded to the influence of refined education on minds of peculiar structure, reason and experience both forbid us to expect that national morality can prevail in exclusion of religious principle. It is substantially true, that virtue or morality is a necessary spring of popular government. The rule, indeed, extends with more or less force to every species of free government. Who, that is a sincere friend to it, can look with indifference upon attempts to shake the foundation of the fabric? (Source: George Washington, Address of George Washington, President of the United States . . . Preparatory to His Declination (Baltimore: George and Henry S. Keatinge), pp. 22-23. In his Farewell Address to the United States in 1796.)