The Lemon test (so-named after [link|http://caselaw.lp.findlaw.com/scripts/getcase.pl?navby=case&court=us&vol=403&invol=602|Lemon v. Kurtzman]) says that the standard for evaluation is this:

  1. Does it serve a secular purpose? Yes. Many religious people are more likely to tell the truth after swearing on the Bible.
  2. Does it have a primary effect of advancing a particular religious belief? No. Requiring it, or treating different religious oaths differently would. But as long as you are neutral, it doesn't.
  3. Does it involve "excessive entanglement" of government and religion? No.


Furthermore tradition has a strong influence. Given that it has been accepted for centuries as part of our legal system, without very specific good cause, it won't change any time soon.

Cheers,
Ben