We take oaths in this country when we testify. Nowadays, anybody can do it, and perjury is the only penalty. Used to be that hellfire also awaited the liar. That was the theory, anyway, and under that theory until well into the 20th Century, the several states wouldn’t let atheists testify because the atheists didn’t believe in future rewards and punishments. I don’t know what the states did about Universalists or cheap grace evangelicals, or even what they’d do with us–our discussion below about having too little hellfire in our beliefs got me wondering, though I eventually concluded that one-thousand years of suffering sufficed. You can see how difficult this could all get (‘Before you take the oath, can you tell us whether you think you can sin a little and God will just punish you with a few stripes?’ ‘Is it your belief that you can stop sinning and repent whenever you want to?’ ‘Does your priest usually only impose light penances?’), so you can understand why it’s gone. We both know, too, that keeping atheists out of the witness box looks pretty discriminatory.
Similar reasoning is probably also behind our rule that you can’t question a witness about his religion in order to undermine his credibility. Seems, however, that one can exclude jurors on the basis of being too religious, presumably as long as one discriminates equally against the faithful of all sects.