The Act of Settlement was passed in 1701. It came into being at the end of the reign of King William III, when his successor to the throne, Anne, had lost her only surviving child. It was felt expedient by Parliament, at that time, to bring into law a requirement that only Protestants, in communion with the Church of England, should be allowed on the throne.
This clause was not included out of spite, or prejudice, or from a sectarian standpoint. Our Protestant forefathers had learned by bitter experience that freedom and liberty are in danger when Romanism is in the ascendency. It is even worse when they are in power. The opening words of the clause emphasis this: : Whereas it has been found by experience… It is essential that we understand the underlying motivation in this Constitutional settlement. The title of the document gives us the key to understanding the matter. It is an Act: declaring the Rights and Liberties of the Subject and settling the Succession of the Crown. The Act of Settlement is there to protect the liberties and freedoms of the citizen. This is the reason for the prohibition.
There is still a desire to bring the British crown back under the rule of the Pope. KT is unfinished business on Rome's part. For this reason also we should oppose a papal visit.