I wondered about whether jurors in the UK have to swear on bibles or what. The Current Mrs Latsot knows about this kind of stuff and sent me the answer (from Wikipedia):
Two things strike me. The first is that atheists and agnostics are bundled with Jehova’s Witnesses, which is simply extraordinary. The second is that swearing on holy books is given consideration in the legal system. It doesn’t matter what holy book it is. And in the case of us Atheist Witnesses, it doesn’t matter whether we have a holy book at all. But as long as people swear on such a thing, there’s apparently a greater chance that they’re telling the truth.
The very idea of promising to tell the truth is embarrassing, much more so when it invokesreligion.