If the Founding Fathers had wanted religion to not be a part of government, they would have not had prayers at the initial confessional meetings. Today, every session of Congress and the Supreme court meetings begins with a prayer. The Founder fathers were not atheist but consisted of a diverse group of different religions who intended to keep religion in government, but you had your choice of which religion you wanted to be affiliated with.
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This is my favorite because the fundies can't try an interpret it to mean something it does not.
Treaty of Peace and Friendship Between the United States and the Bey and Subjects of Tripoli of Barbary, 1796-1797
As the government of the United States of America is not in any sense founded on the Christian Religion--as it has itself no character of enmity against the law, religion or tranquility of Musselmen [Muslims], ... ("Article 11, Treaty of Peace and Friendship between The United States and the Bey and Subjects of Tripoli of Barbary,"