"The Senators and Representatives before mentioned, and the Members of the several State Legislatures, and all executive and judicial Officers, both of the United States and of the several States, shall be bound by Oath or Affirmation, to support this Constitution; but no religious Test shall ever be required as a Qualification to any Office or public Trust under the United States." - Article VI, Clause 3 of the US Constitution.
"I, [NAME], do solemnly swear (or affirm) that I will administer justice without respect to persons, and do equal right to the poor and to the rich, and that I will faithfully and impartially discharge and perform all the duties incumbent upon me as [Associate Justice of the US Supreme Court] under the Constitution and laws of the United States. So help me God." - Oath of Office for a Supreme Court Justice.
The framers of the Constitution rightly prohibited any formal religious test for holding office. Anyone of any faith or even of no faith is legally eligible for public office. This provision avoids some of the uglier practices in Great Britain that the framers knew tied Church to State in ways that resulted in vicious internal conflict and threatened religious liberty.
Even though there is no formal religious test for office, some on the Left have questioned the suitability of Justice John Roberts, Jr. for the Supreme Court because he takes his Catholic faith seriously. The implicit assumption is that a Catholic can not render a judicial decision with respect to abortion consistent with the law since the Catholic Church has a strong position against abortion. Make no mistake. The question is primarily about abortion.
Questions about Roberts' religion on the Left, from the likes of Christopher Hitchens and E. J. Dionne are only being broached because of abortion. The Catholic Church is also strongly (though not as strongly) against capital punishment. However, the legal status of capital punishment under a Catholic judge is not the issue that worries Dionne or Hitchens. Moreover, if a Conservative Protestant where to question the qualifications of a Catholic judge because of the Church's stand against capital punishment, he or she would be loudly and properly chastised for religious intolerance. However, in the service of abortion, the Left and the media that support the Left, have difficulty in recognizing any limits of probity.