Can an atheist become President of the United States and take the oath on the Bible?
An atheist or agnostic can become President and NOT take the Oath on the Bible. There is no requirement for a Bible in the taking of the Oath, and although it is often included the required wording of the Oath does not include the formulation “so help me God”. The wording is here: http://www.law.mq.edu.au/Units/law404/US%20Presidential%20Oath%20of%20Office.htm It’s called an “oath or affirmation”, and I think someone of no clear faith would simply affirm rather than swear their intention to uphold the office.
