Ever since the English Reformation of the 16th Century, British Protestants have harboured a suspicion of Catholicism. Even today, the Pope's overtures towards the more conservative clergy in the Church of England have raised hackles across the country.
The ascension to the throne of Elizabeth I (1533-1603), considered “The Child of the Reformation,” seemed to confirm England as a Protestant country. But throughout her reign, Elizabeth faced Catholic conspiracies and attacks from within and outside her kingdom. The focus of political opposition to Elizabeth was her Catholic cousin, Mary I of Scotland (1542-1587), who was imprisoned and ultimately executed on Elizabeth's orders.
The old religion of Catholicism came to be seen as such a threat to the security of the realm that the return of Catholics to the throne in the person of Charles I (1600-1649) and his son James II (1633-1701) led to both kings being forcibly removed -- the former by civil war and the latter by foreign invasion. After James II was deposed in favour of his Protestant daughter Mary II (1662-1694) and her Dutch husband William III (1650-1702), it was written into law that the British monarch could not be Catholic or marry a Catholic. As late as the twentieth century, Catholics were barred from various sectors of British life, including some of the best schools. Technically, the Prime Minister of Great Britain may not be a Roman Catholic; Tony Blair converted to Catholicism only on leaving office.