Milan is a main city in the Po valley, at the North of Italy. Originally founded by the Celts around 600 BC and was conquered by the Romans around 222 BC, who gave it the name of Mediolanum. The city was the capital of the Roman Empire for a short time in the Fourth century. This area was conquested by Ostrogoths, Lombards and Byzantines, prior Charlemagne defeat the last Lombard king and subsequently the city became part of the Germanic Holy Roman Empire as a semi-republic.
Historically the Milanese church has been in full communion with Rome, recognizing it and its bishops. The Milanese Bishop (honorly called the Patriarch of Milan) is an equal to the Bishops of the World, be it of Rome or Constantinople.
Among its bishops should be named St. Protasius, St. Eustorgius and St. Dionysius, who firmly opposed apostasy imposed by the Roman Emperor Constantius II. St. Dionysus was exiled to Cappadocia (355), while the Romans put the heretic Auxentius on the episcopal throne of Milan. But many people remained faithful to the original faith. At the death of Auxentius, the great St. Ambrose was elected bishop by the people of Milan (374-97), and was the guide of princes Gratian, Valentinian II, and Theodosius. He was succeeded by St. Simplicianus (397), and Venerius (400); Lazarus (438-49) appears to have amplified the Ambrosian rite of Milan; St. Datius (530-52), lived almost always in exile at Constantinople, on account of the Gothic War; Vitalis (552) reaffirmed the independence of the Milian Church in relation of Rome, but Auxanus (556) re-established the yoke with Rome.
Councils were held at Milan in 343 and 347, against Photinus; in the cause of St. Athanasius, at which the Emperor Constans menaced the bishops; 390, against Jovinian; 451, against the Robber Council of Ephesus; 680, against the Monothelites; 1060, 1098, 1117, 1287, for ecclesiastical reforms.
The Frankish king Childebert gave to Bishop Laurentius II the title of Patriarch.
The domination of Rome brought corruption to the Milanese church, but such men as the priest Anselmo da Biaggio, later Bishop of Lucca, and the cleric Arialdo, used force to compel the clergy to observe continence, and to drive its members from benefices obtained by simony. From this great confusion ensued. In 1059 Nicholas II sent to Milan St. Peter Damian and the same Anselmo, at which the people murmured, demanding that the church of Milan be not subject to that of Rome. The Roman Archbishop Guido, however, promised amendment, and accepted the conditions imposed upon him, but soon relapsed, and Arialdo was brutally assassinated in 1065. The people of Milan revolted and Guido was compelled to leave the city, but before he sold his see. Until 1085 there were several pretenders to the see, but finally the Romans took control of the church.
Some of the Milanese became Roman popes, like Uberto Crivelli Pope Urban III in 1185 and Pietro Filargo (1402), who became Alexander V. Another Milanese, Charles Borromeo battle for the reformation of morals. The city elected without the consent of the Vatican, Filippo Visconti (1784-1801), under protection of the Austrian Emeperor Joseph II. Rome later appointed the Cardinal Caprara, who was an ally of Napoleon I till his death in 1811. Since then the Milanese Church has been without (in the Milanese Apostolic Church eyes) a legitimate Bishop and Patriarch, because Rome put Cardinal Carlo Gætano Gaisruck in 1818 and governed the diocese until 1848 "more as a soldier than as a prelate". Some did not recognized the Roman archbishops and led it to break away from the Roman Church.