Thomas Kidd, professor of history at Baylor University and the author of God of Liberty: A Religious History of the American Revolution, said "the founders were certainly aware of other religions besides Christianity, and discussed them at length in their writings."
Kidd pointed us to a 1818 letter from John Adams: "This country has done much. I wish it would do more; and annul every narrow idea in religion, government and commerce," Adams wrote. "It has pleased the providence of the first cause, the universal cause, that Abraham should give religion not only to Hebrews, but to Christians and Mohomitans, the greatest part of the modern civilized world."
Benjamin Franklin also weighed in on the subject. Jan Ellen Lewis, professor of history at Rutgers University, cited Franklin’s autobiography, when he praised a new meeting house built in Philadephia. [sic]
"The design in building not being to accommodate any particular sect, but the inhabitants in general," Franklin wrote. "So that even if the Mufti of Constantinople were to send a missionary to preach Mohammedanism to us, he would find a pulpit at his service."