JPB wrote: what about the use of the bible for swearing the oath?
As an - perhaps - interesting aside: "to swear on something" is the Germanic background of the oath - usually it was a sword, could have been anything else, though.
The reason was that by touching this sword you got connected to the powers of oath, so the oath only became valid after doing such.
The (Irish) missionaries only changed the item ... like with various other Germanic customs.