The old testament is Jewish.

Why not require a new legislator to take the oath on an Archie comic book?

Or the latest issue of Popular Mechanics?

Amendment I

Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances.

Just exactly of what relevance is swearing oaths on other people's holy books to holding public office in America anyway?