The only time I was on the stand in a court of law, I refused to swear on the "holy" bible. After a bit of initial confusion the judge allowed me to make a promise that I would tell the truth, the whole truth, and nothing but the truth. I didn't end that with "So help me, God" because, as I explained to the judge, I am an atheist.
(I could have invoked the Almighty Cod - but I didn't think the serious nature of the proceedings warranted that sort of humour.)
(And, no. I wasn't the accused. I was just giving evidence.)