Could the Queen of England convert and become a Roman Catholic and remain the Queen? Does the Bill of Rights of 1689 still prevent a Roman Catholic from being King or Queen?
It is the Act of Settlement 1701 that covers this:
“Only the descendants of Sophia who were Anglican or Protestant, and had not married a Roman Catholic could succeed the throne. Roman Catholics and those married to Roman Catholics are barred from ascending the throne “for ever”.”
and
“That the monarch “shall join in communion with the Church of England””
So, No, unless the Act of Settlement is removed from the Statute Book.
But I think his question is what happens if the current monarch converts to Catholocism after ascending the throne. Would he or she be required to abdicate?
The linked Wikipedia article says that Prince Edward*, the Duke of Kent is still considered to be in the line of succession even though his wife Katherine converted to Catholicism in 1994. But she was an Anglican when they married in 1961. The ruling appears to be that he did not marry a Catholic and therefore did not violate the Act. This would seem to imply that the Act only has an effect at the time of an event (such as a marriage or coronation) and any conversions subsequent to the event do not apply.
*This is the Prince Edward who is Queen Elizabeth’s uncle, not the Prince Edward who is her son.
IMO, the Windsor Precedent would suggest that she would be “advised” to abdicate – if the sense of offense against Catholicism is still as strong in England as it has been in the past; I don’t know. However, AFAIK there is very little on the books that says what a reigning monarch must and must not do.
BTW, Prince Edward, Duke of Kent, is the Queen’s cousin, not her uncle. Prince George, Duke of Kent and brother of King George VI (Bertie to the family) died in a plane crash in 1942, when his son the present Duke was aged seven.
Here’s how a parliamentary research paper research paper summarises the issue:
Note that this paper doesn’t address the OP, of a monarch who inherits and then converts, but I would think that Polycarp has got it right - that if the monarch converted, there would be a real issue whether she still could hold the throne, and would be advised to abdicate. The simplest way to put it is that she would have breached the fundamental compact set out in her coronation act, and therfore her conversion would itself be a de facto abdication.
The other point that isn’t addressed is the marriage of an heir to a Roman Catholic. Does such a marriage act as a lifetime exclusion, or is it only during the time of the marriage? If the Duke of Kent’s wife died, or they divorced, would he still be excluded from the throne?
By the way, minor nitpick to the OP: there is no such person as the Queen of England. The last Queen of England was Queen Anne. Queen Elizabeth II is the Queen of the United Kingdom, Canada, and her other Realms and Territories.
What happens if the Consort is of some religion that’s neither Anglican or Catholic, or even Christian? Say they’re a Buddhist, Muslim, Wiccan, or even atheist. They’re not violating the letter of the law, but surely there’s a strong implication that the Consort should be Protestant or possibly Eastern Orthodox.
The Law of Succession was written in the days when all prospective Consorts (mainly European royalty) were some species of Christian. But things have changed and now there are all manner of other religions with strong followings in the realm. I’d say there’s a significant chance that whoever Prince William marries could be a non-Christian. Will Parliament (the ultimate authority in these matters, I assume) have a problem with an atheist or Wiccan Queen Consort?
Prince Philip was received into the Church of England before he and QEII married.
Probably spurious as the guy was brought up in the UK from about age four 
Must have been some Orthodox thing that led Prince Philip to order MI5 to assassinate Diana, Princess of Wales, a soon as there was the slightest chance she might marry a Muslim 
I seem to remember that Charles recently expressed a desire to be crowned ‘Defender of Faiths’ (plural) due to the multicultural nature of the UK. Unfortunately the idea was shot down by the people who actually keep track of legislation in this area.
(No cite, sorry. Can’t be bothered. Not being a good doper today).
I think it was to change from being “Defender of the Faith” to plain “Defender of Faith”.
Yup, along those lines. I think he got smacked down either way.
The texts of both the Bill of Rights and the Act of Settlement could not be clearer.
If the current monarch converts, they automatically cease to be the monarch and the next valid person succeeds. It’s as simple as that. Although doubtless it would be slightly more complicated in practice.
Moreover, any exclusion is permanent, with someone excluded being ‘for ever uncapeable’ of succeeding.
In both cases, the relevant passage from the Bill of Rights is -
This was reiterated word-for-word by the Act of Settlement, which however went on to add the clarification that the person who thus qualified was the Electress Sophia.
(Incidentally, the Wikipedia article is correct: the Duchess of Kent’s subsequent conversion does not bar her husband from the succession. The Bill of Rights and the Act of Settlement only bar someone who ‘shall marry a Papist’, which is subtlely different from finding yourself married to one.)
Quite right, APB, my mistake. (Note to self - quit checking summaries rather than the actual text…)