In 1915, with departure for France and possible death there imminent, Harold Macmillan, his friend Guy Lawrence, and Macmillan's old tutor Ronald Knox http://en.wikipedia.org/wiki/Ronald_Knox "more or less agreed to be converted to Roman Catholicism." Alistair Horne, *Harold Macmillan: Politician, 1894-1956,* p. 31. Macmillan, however, ultimately decided not to do so. He was perhaps disappointed that Knox had remained at Oxford instead of signing up like him and Lawrence, who would die in battle. (Knox would explain that he had concluded that "the profession of arms was forbidden to the clergy except...under coercion"[1].) But another factor was the fierce Protestantism of Macmillan's mother Nellie, with her Indiana Methodist background. She was distressed enough that Knox, in his pre-Romanist phase, had converted Harold's brother Arthur to *Anglo*-Catholicism...
So let's say that Macmillan does indeed convert to Roman Catholicism. Does this prevent him from later becoming Prime Minister? It does not bar him constitutionally:
"3. Prime Minister
"While there is no longer a statutory bar on Roman Catholics becoming Prime Minister, there would be similar issues [to those involving the Lord Chancellor] relating to the advice on ecclesiastical preferment that is given by the Prime Minister to the Crown. Special arrangements would have to be made to ensure that he or she did not advise the Crown directly or indirectly on Church of England appointments, doing which under the 1829 Roman Catholic Relief Act remains a 'high misdemeanour'. This Act does not, of course, apply to Prime Ministers who are Protestant nonconformists, though in all probability similar arrangements would be made. Unlike the case of the Lord Chancellor, no clarifying legislation exists, but in a similar fashion, this particular aspect of Prime Ministerial duties could be delegated to another Minister.
"All Prime Ministers to date have been Protestants. Disraeli was born into the Jewish faith but was baptised into the Church of England at the age of twelve. The majority of Prime Ministers, including the current one, Tony Blair, have been Anglicans, and Bute was a Scottish Episcopalian. Balfour and Campbell-Bannerman belonged to the Church of Scotland; and Bonar Law and MacDonald were also Presbyterians. Shelburne was a dissenter; Lloyd George and Callaghan were Baptists; Grafton and Chamberlain Unitarians (though Grafton was an Anglican during his tenure as PM); Wilson was a Congregationalist and Thatcher a Methodist." http://researchbriefings.files.parliament.uk/documents/SN01493/SN01493.pdf
(Blair later became a Roman Catholic, but only after he had ceased to be Prime Minister.)
So--no constitutional bar, but I doubt very much that the Conservative Party's influentials in the 1950's were ready for a Roman Catholic as Party Leader and Prime Minister. So does that mean that Rab Butler gets the job instead? Or would the weaknesses that doomed Butler against Macmillan in OTL (Butler's support for Munich, his indecisiveness on Suez, the fact that he was an intellectual who was often unable to hide his contempt for mediocrity, and his lack of a killer instinct) guarantee that someone else would become Prime Minister, and if so who?
[1] *A Spiritual Aeneid*, p. 174 http://books.google.com/books?id=qFQAAAAAMAAJ&pg=PA174
So let's say that Macmillan does indeed convert to Roman Catholicism. Does this prevent him from later becoming Prime Minister? It does not bar him constitutionally:
"3. Prime Minister
"While there is no longer a statutory bar on Roman Catholics becoming Prime Minister, there would be similar issues [to those involving the Lord Chancellor] relating to the advice on ecclesiastical preferment that is given by the Prime Minister to the Crown. Special arrangements would have to be made to ensure that he or she did not advise the Crown directly or indirectly on Church of England appointments, doing which under the 1829 Roman Catholic Relief Act remains a 'high misdemeanour'. This Act does not, of course, apply to Prime Ministers who are Protestant nonconformists, though in all probability similar arrangements would be made. Unlike the case of the Lord Chancellor, no clarifying legislation exists, but in a similar fashion, this particular aspect of Prime Ministerial duties could be delegated to another Minister.
"All Prime Ministers to date have been Protestants. Disraeli was born into the Jewish faith but was baptised into the Church of England at the age of twelve. The majority of Prime Ministers, including the current one, Tony Blair, have been Anglicans, and Bute was a Scottish Episcopalian. Balfour and Campbell-Bannerman belonged to the Church of Scotland; and Bonar Law and MacDonald were also Presbyterians. Shelburne was a dissenter; Lloyd George and Callaghan were Baptists; Grafton and Chamberlain Unitarians (though Grafton was an Anglican during his tenure as PM); Wilson was a Congregationalist and Thatcher a Methodist." http://researchbriefings.files.parliament.uk/documents/SN01493/SN01493.pdf
(Blair later became a Roman Catholic, but only after he had ceased to be Prime Minister.)
So--no constitutional bar, but I doubt very much that the Conservative Party's influentials in the 1950's were ready for a Roman Catholic as Party Leader and Prime Minister. So does that mean that Rab Butler gets the job instead? Or would the weaknesses that doomed Butler against Macmillan in OTL (Butler's support for Munich, his indecisiveness on Suez, the fact that he was an intellectual who was often unable to hide his contempt for mediocrity, and his lack of a killer instinct) guarantee that someone else would become Prime Minister, and if so who?
[1] *A Spiritual Aeneid*, p. 174 http://books.google.com/books?id=qFQAAAAAMAAJ&pg=PA174