In 1776, John Adams wrote, "All sober enquiries after truth, ancient and modern, Pagan and Christian, have declared that the happiness of man, as well as his dignity consists in virtue. Confucius, Zoroaster, Socrates, Mahomet, not to mention authorities really sacred, have agreed in this."
Adams did not believe in Zoroastrianism and Islam, but he respected them as serious "enquiries after truth...." Philip B. Kurland and Ralph Lerner, eds., The Founders’ Constitution 5 vols. (Chicago: University of Chicago Press, 1987), 1:108.
For more information, please see James H. Hutson, "The Founding Fathers and Islam: Library Records Show Early Tolerance for Muslim Faith," The Library of Congress Information Bulletin Volume 6 Number 5 (May 2002), http://www.loc.gov/loc/lcib/0205/tolerance.html (accessed 2 January 2012)