According to its most commonly repeated history, the original institution from which the modern Order descends was founded by the Emperor Constantine the Great to provide a guard for the Labarum, or Standard, which had been carried at the head of his troops at the Battle of the Milvian Bridge in 312 a.d. [Note 1.2] This great victory over the pagan forces of Maxentius led to the conversion of the Roman Empire to Christianity and had a profound effect on European history. It probably seemed appropriate to sixteenth and seventeenth century historians that such a momentous event should have been commemorated by the establishment of an Order of Chivalry. [Note 1.3] Statutes dating from the sixteenth century, state that the original rules were confirmed and amended by the Emperors Isaac Angelus and Michael Paleologus, but the only extant versions of these statutes date from the sixteenth century. The Constantinian Order subsequently received confirmation as a Religious-Military Order from the Holy See and, with the Order of Malta, is the only international Catholic Order which has maintained this status unchanged to the present day. The Order is dedicated to propaganda of the Faith, Defence of the Church and support of the Holy See; the members owe a particular duty of loyalty to the Pope and to support the teachings and dogma of the Roman Catholic Church.
The Constantinian Order is also the only international Catholic Order with an hereditary Grand Mastership. Since 1731 this has been the inheritance of the Bourbon family descended from Philip V of Spain and his wife Elizabeth Farnese [See Bourbon Dynasty]. Although often described as a Neapolitan institution, this is a mistaken view of the Order's history. Before 1698 its administrative headquarters was based in Venice, the residence of its Grand Masters, or traveled with them. From 1698 until 1768 it was based in Parma, even though the Grand Master himself took up residence in Naples in 1734. The administration was removed Naples and the separate administration of the Order's Parmesan properties terminated in 1780; aside from the period of the French occupation from 1797-99 and 1806-1815, the administration remained in Naples until the downfall of the Two Sicilies Monarchy in 1860-61. Forced into involuntary exile from his homeland the Grand Master moved to the Palazzo Farnese in Rome. The nominal administration remained in Rome until 1960, even though the Grand Masters themselves were resident in Bavaria and Cannes, and a Constantinian chapel was built in the Basilica of Santa Croce al Flaminio and dedicated by Pope Benedict XV. His Holiness Pope John Paul II celebrated Mass there on February 23, 1997; HRH the Grand Master attended, accompanied by the Spanish Ambassadors to the Holy See and the Quirinale and the President of the Italian Commission of the Order, along with a number of Constantinian Knights. This Church is also the site of the annual Saint George's Day celebrations and a monthly Mass for Knights and Dames of the Order.
The preponderance of members from Southern Italy contributed to the belief that it is a Neapolitan institution and, indeed, from the last quarter of the eighteenth century onwards most of the senior posts in the administration and a majority of the knights were drawn from the nobility of the (former) Kingdom of the Two Sicilies. The Two Sicilies Crown and Constantinian Grand Magistery, however, are entirely independent of each other even though they were united in the same person. The Constantinian Grand Magistery is an inheritance by male primogeniture from the Farnese dynasty. On the extinction of the latter, the succession passed with Papal assent to the nearest male heir, the Infante don Carlos de Borbón y Farnese (later Charles III of Spain). In 1759, after the latter transferred his Neapolitan Crown to his third son Ferdinand, the Constantinian Grand Magistery was ceded in a separate act ten days later, the young King Ferdinand being declared "legitimate primogeniture male heir of the Farnese".
The succession to the Constantinian Grand Magistery can only pass to males and, on the extinction of the last male descendant of Charles III, it passes to the Bourbon-Parma line. The Two Sicilies Crown, however, must pass to the nearest female heiress of the last male descended from Charles III. Thus, even though the separation of the two successions may seem unlikely, the very fact that they pass by different laws is evidence of their mutual separation. Every historian of the Order without exception, writing between 1860 and 1960, emphasized the complete independence of the two successions. Indeed, it was the fact that the Grand Magistery was independent of the Crown which led the Italian Government to determine that it was excluded from those acts suppressing the Two Sicilies Crown and Orders. Had it been part of the inheritance of the Crown of the Two Sicilies, it would not have survived 1860 in Italian law. Thus no act or decree concerned solely with the succession to the Two Sicilies Crown can affect the succession by male primogeniture to the Constantinian Grand Magistery. The Order is regulated as a subject of canon law, and the succession to the Grand Magistery is governed first by the statutes of 1705, approved by Papal Brief in 1706 and the Bull "Militantis Ecclesiae" of 1718, and then confirmed in the statutes of 1922 approved by Papal Placet (these statutes modified in 1934, 1943, 1987 and 2004).
1.1. Adapted and updated from the chapters on the Constantinian Order in The Orders of Chivalry and Merit of the Bourbon Two Sicilies Dynasty, by Guy Stair Sainty, Madrid, 1989.
1.2. Bernardo Giustiniani, Compendio, Historico, dell'Ordine, .....Equestre, Imperiale, Angelico Aureato Costantiniano di San Giorgio ..., Venice 1680.
1.3. An alternative hypothesis, proposed without any documentary evidence to support it, was that the original Constantinian knights were the Domestic Protectors of the Imperial Palaces, who had formed themselves into a confraternity at the time of the fall of Constantinople.