Can they constitutionally do so? Surely such a clause would be an establishment of religion, banned by the Federal constitution. And where would agnostics come in? And how would you stop spoof religions, such as Pastafarianism?

There are actually quite a few states with such language in their constitution. The ones I could dig up are: Texas Tennessee South Carolina North Carolina Arkansas Maryland Pennslyvania Such a statement is plainly unconstitutional, but in most cases it has not been challenged. I think the Maryland provision has been struck down in court. Here's exactly what Tennessee's constitution says: ARTICLE I Declaration of Rights Section 4. That no political or religious test, other than an oath to support the Constitution of the United States and of this state, shall ever be required as a qualification to any office or public trust under this state. BUT then it also says: ARTICLE IX Disqualifications Section 2: No person who denies the being of God, or a future state of rewards and punishments, shall hold any office in the civil department of this state. ****** Article 19, § 1 of Arkansas's constitution provides: No person who denies the being of a God shall hold any office in the civil departments of this State, nor be competent to testify as a witness in any court. ********* Article VI, § 2 of South Carolina's constitution says this: No person who denies the existence of the Supreme Being shall hold any office under this Constitution. ******** North Carolina "The following persons shall be disqualified for office: First, any person who shall deny the being of Almighty God...." Constitution Article 6 Section 8 ********** Texas "No religious test shall ever be required as a qualification to any office, or public trust, in this State; nor shall any one be excluded from holding office on account of his religious sentiments, provided he acknowledge the existence of a Supreme Being." Article 1 - Bill of Rights: Section 4 Double check that last sentence - contradictions ahoy!! ******** I suppose this would be part of the reason why, in American history, there has been only ONE politician in office to ever identify publicly as an atheist. He is a congressman from California. So, yeah, nice to know that not only do I have no representation in government whatsoever, it would actually be illegal in several states for anyone like me to hold office. :(