This was George Washington's response when asked why there were no mentions of, nor requirements pertaining to religion or Christianity included in the Constitution:
October 1789
To the Ministers and Ruling Elders delegated to represent the churches in Massachusetts and New Hampshire, which compose the First Presbytery of the Eastward.
The tribute of thanksgiving which you offer to the gracious Father of lights, for his inspiration of our public councils with wisdom and firmness to complete the national Constitution, is worthy of men who, devoted to the pious purposes of religion, desire their accomplishment by such means as advance the temporal happiness of mankind. And here, I am persuaded, you will permit me to observe, that the path of true piety is so plain as to require but little Political attention. To this consideration we ought to ascribe the absence of any regulation respecting religion from the Magna Charta of our country.
From the Treaty of Peace and Friendship Between the United States and the Bey and Subjects of Barbary, June 10, 1797
This treaty was made under the administration of George Washington, and was signed and sealed at Tripoli on the fourth Day of November 1796, and at Algiers the third day of January 1797, by Hassan Bashaw, Bey of Algiers, and Joel Barlow, Counsul- General of the United States. This treaty was ratified by the U.S. Senate June 7, 1797 and signed into law by President John Adams June 10, 1797.
ARTICLE 11. 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 tranquility, of Mussulmans; and, as the said States never entered into any war, or act of hostility against any Mahometan 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.