The announcement of an agreement amongst the governments of all Her Majesty's realms to amend the law regarding the Succession, as reported here and here raises several thoughts.
The process reflects a mood of a renewed, positive view of the Monarchy in the wake of the wedding of the Duke and Duchess of Cambridge, and notably in some of the leading Commonwealth realms - as detailed for Canada in The Monarchist blog and it can be seen, in a slightly quirky way, in the response to Miss Gillard not cutseying to her Queen in Australia. Such a trend in opinion is good in itself.
Retaining an insistance on the Monarch not being Catholic may be politically and ecclesially judicious at present, but given that the Sovereign's role as Supreme Governor in practical terms is fornally issuing the conge d'elire to elect Bishops and receiving their homage, and signing into law the Measures which having passed unchanged through Parliamnet representing the latest daft ideas of the General Syond it is no different from their position as Monarch in temporal affairs. So a Catholic could do that as well as an extension of the Monarch's duties. That is not to say that the Monarch does not have an important part to play by their personal support of Christianity - The Queen has been exemplary in this matter.
In reality the last time such a legal provision would have affected the succession would have been in 1901, and we must presume that had the law been in place in the earlier nineteenth century the Princess Royal would not have married abroad....
Fortunately, and ultimately, it depends on genetics rather than on politicians, and its effective intoduction is at some hypothetical date in the future, and at least two more reigns away.