On Monday, the consistory headed by Pope Francis decided to elevate Müller to the class of cardinal priests at his request, the Vatican announced.
Müller's titular deaconry, Sant'Agnese in Agone, will be temporarily elevated to the status of titular church during his term of office ("pro hac vice").
After ten years in office, cardinal deacons have the right to ask the Pope for elevation to cardinal priest and for a titular church ("optatio").
Müller was appointed to the College of Cardinals on 22 February 2014 into the College of Cardinals.
After his retirement as Prefect of the Congregation for the Doctrine of the Faith Müller is now judge at the Apostolic Signaturethe highest court of the Church.
The College of Cardinals is divided into the three classes of cardinal bishops, cardinal priests and cardinal priests. are organised.
However, the classes are independent of the actual degree of ordination. As a rule, all cardinals are bishops. Cardinal bishops are cardinals who are assigned the title of one of the suburbicarian bishoprics in the neighbourhood of Rome.
However, the leadership of these dioceses is separate from the holders of the titles; they each have their own diocesan bishops. In addition, the patriarchs of the Eastern Catholic Churches who have been elevated to cardinals belong to the cardinal bishops.
Pope Francis has also decreed that the episcopal class be expanded by five further members without a suburbicarian title.
Cardinal priests are cardinals who are assigned a titular church in Rome; cardinal deacons are assigned a diaconate. Müller was the only one of the German cardinals in this class. The shift to the priestly class changes the order of honour, but does not entail any further privileges.
Müller's deaconry of Sant'Agnese in Agone was last the titular church of a cardinal priest from 1517 to 1654, until the title was relinquished. It is only since 1998 that the church has once again held the title of cardinal deacon.
In addition to Müller, Cardinal James Michael Harveythe archpriest of the Patriarchal Basilica of St Paul Outside the Walls, and Lorenzo Baldisseri, the Secretary General Emeritus of the Synod of Bishops, were also admitted to the class of cardinal priests.
Harvey was previously the longest-serving cardinal deacon under the age of 80 and thus had the task of announcing the election of the new pope after a conclave.
This task now falls to the Prefect of the Apostolic Signature, Cardinal Dominique Mamberti.