Boris Johnson married his partner Carrie Symonds at Westminster Cathedral over the weekend. According to a statement from the cathedral, he is now, in fact, a Roman Catholic. This simple statement raises awkward constitutional questions.
The Sovereign is formally responsible for making a number of appointments within the Church of England, and since the 18th century those powers have usually been exercised on the Prime Minister’s advice. While the 20th century saw the Church gain greater control over ecclesiastical appointments, Downing Street still remains part of the process.
In the case of bishops, the Prime Minister’s role is limited. The Crown Nominations Commission gives Downing Street a single name and the Prime Minister invariably recommends that person to the Queen. However, that arrangement is not enshrined in law. In theory, there is nothing stopping Johnson or any other Prime Minister from rejecting the CNC’s nominee or asking them to provide him with more than one name.
Downing Street retains a more substantial role in making other appointments. For example, No. 10’s Honours and Appointments Secretariat is in charge of finding candidates to fill Crown livings (i.e., those benefices where the Queen is patron).
Johnson’s religious affiliation is an issue because section 18 of the Roman Catholic Relief Act 1829 states that:
It shall not be lawful for any person professing the Roman Catholic religion directly or indirectly to advise his Majesty, or any person or persons holding or exercising the office of guardians of the United Kingdom, or of regent of the United Kingdom, under whatever name, style, or title such office may be constituted, or the lord lieutenant of Ireland, touching or concerning the appointment to or disposal of any office or preferment in the Church of England, or in the Church of Scotland; and if any such person shall offend in the premises he shall, being thereof convicted by due course of law, be deemed guilty of a high misdemeanor, and disabled for ever from holding any office, civil or military, under the Crown.
Roman Catholics and Jews are the only non-Anglicans to be legally barred from advising the Sovereign on ecclesiastical appointments. There is no such disability for Muslims, Buddhists, Atheists, or Lutherans.
Given the reality of the modern Church/State relationship in Britain, it’s debatable whether these prohibitions are still justified. It’s hard to envision a Roman Catholic Prime Minister trying to install a Jesuit in Canterbury Cathedral. Even if they tried to do something like that, there would be other impediments to stop them. And if these prohibitions are deemed necessary to protect the Church, then they should arguably be extended to all non-Anglicans, not just Roman Catholics and Jews.
Until Parliament changes the law, there is an easy workaround: Johnson can simply allow another Minister of the Crown to advise the Queen on ecclesiastical appointments. The flexibility of the British constitution would be an asset here—since the Prime Minister’s role in church appointments is a matter of convention rather than law, there’s no legal obstacle to that kind of delegation. Alternatively, Parliament could explicitly authorize such an arrangement like they did for the office of Lord Chancellor.
 While some have proposed allowing Church figures to advise the Queen on ecclesiastical appointments directly, this wouldn’t be compatible with the precept that a constitutional sovereign always acts on the advice of responsible ministers.
 The prohibition on Jews advising the Monarch is contained in section 4 of the Jews Relief Act 1858. It’s not clear if a secular Jew would fall under the prohibition but presumably that would be the case.
 For starters, the Queen would be well within her rights to refuse to accept the Prime Minister’s recommendation to appoint a Roman Catholic to a post in the Church of England since doing so would be against the law.
 Interestingly, while the Prime Minister is not required to be an Anglican, the Prime Minister’s Appointments Secretary is.
 Section 2 of the Lord Chancellor (Tenure of Office and Discharge of Ecclesiastical Functions) Act 1974 allows the Queen to allow another Minister of the Crown to carry out the Lord Chancellor’s ecclesiastical functions if the office is held by a Roman Catholic.