By Robert Pigott
Religious affairs correspondent
Roman Catholics believe the Pope has authority over their own monarchs
The general support reflected in the BBC's poll for allowing the heir to the throne to marry a Roman Catholic reflects the huge progress made by the Church in moving into the mainstream of British society in the last few decades.
It now seems amazing that when the Queen's father George VI was crowned at Westminster Abbey in 1937, a substantial delegation of Vatican dignitaries including bishops and archbishops, could not be allowed into the building for the service.
Instead a canopy - known as a tribunal - had to be put up outside to shelter the Pope's representatives, and Catholic bishops and archbishops from the UK.
Now, in the cause of ending precisely that sort of discrimination, there seems to be a real prospect of a Roman Catholic sitting inside the Abbey watching his or her spouse being crowned.
But some are questioning whether the nature of the Roman Catholic Church, and what it believes, could actually add to discrimination.
Protestants claim that a Catholic monarch might defer to the Pope as an authority, even in the UK
They cite the requirement placed on the Roman Catholic partner in a mixed marriage to ensure that any children be brought up as Catholics.
A decree issued by the Vatican in 1907, "Ne Temere", reinforced the point, and remains in place.
Does that not make it inevitable, they ask, that there will eventually be Catholic heirs to the throne?
'Ruler of the World'
Liberal Democrat MP Evan Harris is not seeking to change that part of the law that prohibits a Roman Catholic from becoming the monarch, but what would be the public reaction to a Roman Catholic being denied their "birthright" purely on the basis of their religion?
The Queen is the Supreme Governor of the Church of England
Protestant opponents of Dr Harris's proposals say that public pressure would make the next step - allowing Roman Catholics to become monarch - inevitable.
And that, they insist, would lead logically to a "Roman Catholic only" monarchy?
Others might ask whether that would matter.
One complication is the role of the monarch as the Supreme Governor of the Church of England, which describes itself as a "Catholic Church reformed in the Protestant tradition", and many of whose members see themselves as rather more Protestant than Catholic, and who owe no allegiance to the Pope.
"Our position is simple: you need to pay a lot of care and attention when unpicking parts of this nation's constitution, to make sure you don't create unintended consequences," said a Church of England spokesman.
Some Protestant opponents of a change in the Act of Settlement point to a difference between the concept of monarchy between them and the Roman Catholic Church.
At the coronation service the monarch is asked: "Will you to the utmost of your power maintain in the United Kingdom the Protestant Reformed Religion established by law?"
The monarch's jurisdiction is seen as limited to his or her own realm, in the Queen's case to the UK or countries where she is head of state.
Perhaps the Vatican could be persuaded to grant an exemption to the monarch from the rule about raising children as Roman Catholics
But at the Pope's coronation the following words are said to him: "Receive the Tiara adorned with three crowns, and know that thou art Father of Kings and Princes, Ruler of the World, and Vicar on Earth of Jesus Christ."
The statement reflects the Roman Catholic belief in the Pope's international authority - including over monarchs in their own countries.
Protestants claim that a Catholic monarch might defer to the Pope as an authority, even in the UK.
Perhaps the Vatican could be persuaded to grant an exemption to the monarch from the rule about raising children as Roman Catholics.
The Pope might feel that a change in law to end one form of discrimination merited a concession from him to avoid creating another.