I would say it applies to anything, though, not just religion. How can you publicly profess something that violates the principles you claim to have? My point in another post was that even if these reporters were complete secularists and not Christian, by professing Islam they were accepting sharia and rejecting all of the principles of Western liberalism that they claim to believe (freedom, equality, etc.).
Young Conservative, you seem to think that beliefs have no consequences and no substance, hence it does not matter what you publicly profess. Why don't you give this matter an objective test: look at the results. Islam's ideal is theocracy, so are no functioning Islamic secular states (except for those that are run as police states and are pretty tenuous, such as Turkey), the condition of women in Islam is appalling, and Islam has a history of conquest and violence since its very beginning. Christianity, on the other hand, is not a political system, can operate in any political system that respects human rights, has at its heart the fundamental belief in equality and dignity of all human beings, and has a history of building hospitals and caring for lepers and the outcast. A newly converted Christian will go out and want to join a religious order and spend his days in prayer or tending the sick; a newly converted Muslim hits the internet to look for bomb recipes. There's a difference, and you have to admit that what one believes is important and does make a difference on a scale that goes beyond the merely personal.
Again, this is true in anything, not just religion. You obviously do not believe that truth exists; you believe that it is all the same. It's not, and at some point, we have to decide what is our sticking point; that is, what is the thing we cannot deny without sacrificing our integrity, our freedom and our human dignity. What would it be for you? Would you be willing to say anything to stay alive or even to avoid serious difficulties, such as loss of income or status? Or are there any beliefs and values you would refuse to deny, or some you would refuse to profess because they are so opposed to what you really believe?
Then why can't a Roman Catholic become king/queen of England?
Actually most of the problem lay within the root causes of the Protestant Reformation. Thumbnail sketch time ...
Henry VIII, sort of wanted a divorce but he also saw the wealth of the Church which he had no way to get his greedy, little paws on. Resolution, start a new church with monarchy as the head. More money for the crown.
Martin Luther, disgruntled employee, goes "whistleblower" on the Church, forces a diet of worms down the Teutonic gullet. Princes and low monarchy see the opportunity to get their grubby little paws on the Church's property and wealth, convert, make their vassals convert. Resolution, Martin Luther marries a trampy nun, Princes get loot, Germany gets new religion.
Swiss Canton's Taliban won't let John Calvin join. Calvin says, "Screw this!" you can be as good as you want, fate says only 120,000 of all humanity of all time get into Heaven. Resolution, people say, "If my behavior doesn't dictate my getting a cloud and harp, I'll do what I want."
Just remember this and here's the dirly, little secret your Evangelical friends won't tell you, Catholics believe themselves to be Christians but the Protestants don't think that!
I don't think the opinions of Protestants on Catholics are particularly relevant to this case. Christianity is not political at its foundation, but only when various secular forces try to use it in that way; Islam, on the other hand, sees itself as a legal and political system.
The only relevance of what you have posted is that both Catholics and Protestants at various times refused to deny what they believed (even though in many cases the differences were pretty trivial, by today's understanding). In other words, they had certain things that they would not assent to, because their integrity as human beings forbade them to give even the appearance of denial of their principles. We no longer seem to think this matters.