I liked this short article by Christopher Hitchens, inspired by the melee surrounding Rep. Keith Ellison's determination to take his oath of office upon the Quran:

As to the invocation of Jefferson, we know that when he and
James Madison first proposed the Virginia Statute on Religious Freedom (the
frame and basis of the later First Amendment to the Constitution) in 1779, the
preamble began, "Well aware that Almighty God hath created the mind free."
Patrick Henry and other devout Christians attempted to substitute the words
"Jesus Christ" for "Almighty God" in this opening passage and were
overwhelmingly voted down. This vote was interpreted by Jefferson to mean that
Virginia's representatives wanted the law "to comprehend, within the mantle of
its protection, the Jew and the Gentile, the Christian and Mahomedan, the
Hindoo, and Infidel of every denomination." Quite right, too, and so far so
good, even if the term Mahomedan would not be used today, and even if
Jefferson's own private sympathies were with the last named in that list.

A few years later, in 1786, the new United
States found that it was having to deal very directly with the tenets of the
Muslim religion. The Barbary states of North Africa (or, if you prefer, the
North African provinces of the Ottoman Empire, plus Morocco) were using the
ports of today's Algeria, Libya, and Tunisia to wage a war of piracy and
enslavement against all shipping that passed through the Strait of Gibraltar.
Thousands of vessels were taken, and more than a million Europeans and Americans
sold into slavery. The fledgling United States of America was in an especially
difficult position, having forfeited the protection of the British Royal Navy.
Under this pressure, Congress gave assent to the Treaty of Tripoli, negotiated
by Jefferson's friend Joel Barlow, which stated roundly that "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 Musselmen." This has often been taken as a secular
affirmation, which it probably was, but the difficulty for secularists is that
it also attempted to buy off the Muslim pirates by the payment of tribute. That
this might not be so easy was discovered by Jefferson and John Adams when they
went to call on Tripoli's envoy to London, Ambassador Sidi Haji Abdrahaman. They
asked him by what right he extorted money and took slaves in this way. As
Jefferson later reported to Secretary of State John Jay, and to the Congress:

The ambassador answered us that [the right] was
founded on the Laws of the Prophet, that it was written in their Koran, that all nations who should not have answered their authority were sinners, that it was their right and duty to make war upon them wherever they could be found, and to make slaves of all they could take as prisoners, and that every Mussulman who should be slain in battle was sure to go to Paradise.

Medieval as it is, this has a modern ring to it.

I thought at the time that it was not too reasonable on the part of those who criticized Ellison to demand he take his oath on the Christian Bible. It would have been a self-vitiating gesture to do so. You swear on something that you hold sacred and unviolable. Not on someone else's sacred and unviolable something. Had he taken an oath upon a book that had no moral prerogative for him, the ritual of being sworn in would have had no binding postulation upon his conscience. As though he did not take that oath at all.

He could, of course, have opted for "upon my word of honour".

I have wondered, though, what is the legal meaning of taking an oath upon a holy book. There is no Bible that is fully compatible with the democratic ideals of the modern state. How can this oath, then, be comfortably and rationally aligned with such a foundational document as the American Constitution, for example.