So much for the idea that our country's founders intended this to be a Christian Nation. The men who wrote our constitution were men of the Enlightenment period. Recently, I read the book,Freethinkers: A History of American Secularism
At the time of the drafting of our constitution, each state had its own laws regarding who could hold office based on religious affiliation. Some would allow Jews to hold office, but no Catholics, some were vice versa. Yet, the constitutional authors based their document on the state constitution of Virginia, which allowed rights for all beliefs and even those without belief.
One thing that informs secularism of that time was the history of religion and government in Britain and Europe. Princes of Europe would hold one sect as official and demand fidelity by force only to switch religious alliances to make former adherants into heretics. Much of the instability of government ruled by Churches and religion drove our ancestors to the New World. Religious and non-religious alike wanted to be left alone.
The key to freedom of belief and conscience is having a secular government which doesn't choose sides and treats all of its citizens equally despite what they believe. An American citizen needs to walk into a courtroom, city hall, or the halls of the legislature believing that they are represented and accepted regardless of their religion. Christians would feel out of place if there were Quotes from Allah in Arabic inscribed in stone as the walk through a courtroom, city hall, or the legislature, wouldn't they? The same holds true when citizen non-Christan citizens are confronted with Christian inscriptions or a post of the 10 Commandments.
I am currently reading The End of Faith: Religion, Terror, and the Future of Reason