Before the legalization of Christianity in Rome the tituli were private buildings used as Christian churches—also called domus ecclesiae or "house churches"—and took the name of the owner of the building, either a wealthy donor, or a presbyter appointed by the church to run it. For instance, the Titulus Aemilianae, now the church of the Santi Quattro Coronati, drew its name from its foundress, who doubtless owned the extensive suburban Roman villa whose foundations remain under the church and whose audience hall became the ecclesiastical basilica. The most ancient reference to such a Roman church is in the Apology against the Arians of Athanasius in the fourth century, which speaks of a council of bishops assembled "in the place where the Presbyter Vito held his congregation".
By the end of the 5th century they numbered 25, as is confirmed by the Liber Pontificalis. The same number, though with different identities, is given in the reports of councils held in Rome in 499 and 595. In 1120, the number is given as 28. Many more have received the status of titular churches in modern times, other were abandoned, or assigned to another order of cardinals (from deaconry to priestly title or vice versa, permanently or pro hac vice), just for the duration of one incumbent's cardinalate.
In 1059, the right of electing the pope was reserved to the bishops of the seven suburbicarian sees, the priests in charge of the titular churches, and the clergy in charge of the deaconries. These were known collectively as the cardinals.
Accordingly, as ecclesiastics from outside Rome came to be appointed cardinals, they were assigned theoretical responsibility for certain Roman parish churches, a legal fiction establishing their position within the Pope's diocese of Rome. They had no obligation to reside in Rome, and so were not personally responsible for the pastoral care of the titular churches assigned to them, a practice still in force today.
Today, the cardinal priests have a loose patronal relationship with their titular churches, whose cardinal protector they are called. Their names and coats of arms are inscribed on plaques in the churches, they are expected to preach at the church occasionally when they are in Rome, and many raise funds for their church's maintenance and restoration, but they no longer participate in the actual management of the churches. There are (as of 2015) 160 presbyteral titular churches.
Likewise, the cardinal bishops are given only honorary title to one (or for the Cardinal-dean two, Ostia being granted additionally with that dignity) of the seven suburbicarian dioceses, and the cardinal deacons are given a similar relationship to the churches of their cardinal deaconries, of which there were 67 in 2015.
Many cardinals are assigned to tituli with some connection to their home see or country, such as the national churches in Rome. For example, Jean-Claude Turcotte, former Archbishop of Montreal, was made Cardinal Priest of the Santi Martiri Canadesi (Holy Canadian Martyrs); André Vingt-Trois, Archbishop of Paris, is the cardinal priest of San Luigi dei Francesi (St. Louis, King of France).
Those Patriarchs of the Eastern Catholic Churches who become Cardinal-Patriarchs (individually, not by right of their office) constitute an exception: their own patriarchal see is counted as their cardinal title. They belong to the order of cardinal bishops and, in the order of precedence, come before the cardinal priests and immediately after the cardinals who hold the titles of the seven suburbicarian sees.
In the wider sense, the term titular church is also loosely applied to the deaconries diaconiae in Rome assigned to the cardinal-deacons.
Originally, they were charitable institutions first mentioned in connection with Pope Benedict II (684–685). Pope Adrian I (772–795) fixed their number at 18, a number that remained constant until the 16th century.
Since the proliferation of cardinalates, most are serving another pastoral purpose (such as parish) with a cardinal-deacon (often (arch)bishop with a proper cathedral elsewhere) assigned as almost nominal cardinal-protector.
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