Article XI of the Treaty of Tripoli, officially ratified by the Senate with John Adams signature on 10 June 1797.
"As the Government of the United States of America is not in any sense founded on the Christian religion; as it has in itself no character of enmity against the laws, religion, or tranquillity [sic], of Musselmen [sic]; and as the said States never have entered into any war or act of hostility against any Mehomitan [sic] nation, it is declared by the parties that no pretext arising from religious opinions shall ever produce an interruption of the harmony existing between the two countries."
In a letter to Ezra Stiles Ely, 25 June 1819, Thomas Jefferson wrote:
"You say you are a Calvinist. I am not. I am of a sect by myself, as far as I know."
In his, "A Defence [sic] of the Constitutions of Government of the United States of America" [1787-1788], John Adams wrote:
"The United States of America have exhibited, perhaps, the first example of governments erected on the simple principles of nature; and if men are now sufficiently enlightened to disabuse themselves of artifice, imposture, hypocrisy, and superstition, they will consider this event as an era in their history. Although the detail of the formation of the American governments is at present little known or regarded either in Europe or in America, it may hereafter become an object of curiosity. It will never be pretended that any persons employed in that service had interviews with the gods, or were in any degree under the influence of Heaven, more than those at work upon ships or houses, or laboring in merchandise or agriculture; it will forever be acknowledged that these governments were contrived merely by the use of reason and the senses.
". . . Thirteen governments [of the original states] thus founded on the natural authority of the people alone, without a pretence of miracle or mystery, and which are destined to spread over the northern part of that whole quarter of the globe, are a great point gained in favor of the rights of mankind."
James Madison wrote in his Memorial and Remonstrance against Religious Assessments :
"During almost fifteen centuries has the legal establishment of Christianity been on trial. What have been its fruits? More or less in all places, pride and indolence in the Clergy, ignorance and servility in the laity; in both, superstition, bigotry and persecution."
"What influence, in fact, have ecclesiastical establishments had on society? In some instances they have been seen to erect a spiritual tyranny on the ruins of the civil authority; on many instances they have been seen upholding the thrones of political tyranny; in no instance have they been the guardians of the liberties of the people. Rulers who wish to subvert the public liberty may have found an established clergy convenient auxiliaries. A just government, instituted to secure and perpetuate it, needs them not."
These quotes, and many more, are from Little-Known U.S. Document Signed by President Adams Proclaims America's Government Is Secular.
See also, these previous posts.