When the first Roman Emperor, Augustus (reigned 27 BC – 14 AD), transformed the Roman Republic into the Roman Empire in 27 BC, he reformed the office of Prefect at the suggestion of his minister and friend Maecenas. Again elevated into a magistracy, Augustus granted the praefectus urbi all the powers needed to maintain order within the city but even more so, to the ports of Ostia and the Portus Romanus, as well as a zone of one hundred Roman miles (c. 140 km) around the city. The Prefect was superintendent of all guilds and corporations (collegia), and held responsibility (via the praefectus annonae) for the city's welfare system.
A Prefect maintained direct authority through the cohortes urbanae, Rome’s police force (within independent prefecture - vigiles, praefectus vigilum). The Prefect needfully published the laws promulgated by the Emperor. Gradually, the judicial powers of the Prefect expanded. Eventually there was no appealing the Prefect’s sentencing, except to the Roman Emperor a reach over even all the governors of the Roman provinces. Originally the Prefect’s powers were exercised in conjunction with those of the quaestors, but by the 3rd century, the Prefect’s control was total.
In late Antiquity, the office of the Prefect gained freedom from the emperor's direct supervision as the Imperial court was removed from the city. The office was usually held by leading members of Italy's senatorial aristocracy, who remained largely pagan even after Emperor Constantine's conversion to Christianity. Over the following thirty years, Christian holders were few. In such a capacity, Quintus Aurelius Symmachus played a prominent role in the controversy over the Altar of Victory in the late 4th century.
The urban prefecture survived the fall of the Western Roman Empire, and remained active under the Ostrogothic Kingdom and well after the Byzantine reconquest.
Each of the tetrarchs themselves often were at work within the province or eparchy, while delegating most of the administration to the hierarchic bureaucracy headed by his respective Pretorian Prefect, each supervising several Vicarii, the governor-generals in charge of the civil diocese. The four tetrarchic capitals of the time were Nicomedia in northwestern Asia Minor (modern Izmit in Turkey), a base for defence against invasion from the Balkans and Persia's Sassanids. Sirmium (modern Sremska Mitrovica in the Vojvodina region of modern Serbia, and near Belgrade, on the Danube border) was the capital of Galerius, the eastern Caesar; this was to become the Balkans-Danube prefecture Illyricum. Mediolanum (modern Milan, near the Alps) was the capital of Maximian, the western Augustus; his domain became "Italia et Africa", with only a short exterior border. Augusta Treverorum (modern Trier, in Germany) was the capital of Constantius Chlorus, the western Caesar, near the strategic Rhine border; it had been the capital of Gallic emperor Tetricus I. This quarter became the prefecture Galliae.
The tetrarchy lasted until c. 313, when internecine conflict eliminated most of the claimants to power, leaving Constantine in control of the western half of the empire, and Licinius in control of the eastern half.
Orthodox Nicene Christianity would become the essential to securitise central control at the capital, becoming the official state church of the Roman Empire under Constantine. On taking the Imperial office in 306, Constantine I restored Christians to full legal equality and returned property that had been confiscated during the persecution. The Church of the Holy Sepulchre was built ultimately on Constantine I’s orders at the purported site of Jesus' tomb in Jerusalem and became the holiest place in Christendom, and he is venerated naturally as a Saint. Constantine the son of Constantius I had of course traveled through Palestine at the right hand of Diocletian, and was present at the palace in Nicomedia in 303 and 305. Most notably during Constantine's tenure, he had the Old Saint Peter's Basilica built.
In 325, Constantine I convened the Council of Nicea, effectively the first Ecumenical Council (unless the Council of Jerusalem is so classified). The creed established and affirmed the doctrine that Jesus, the Son, was equal to God the Father, one with the Father, and of the same substance, or co-essential (homoousios in Greek). The Council condemned the teachings of Arius who they declared a heretic, for believing Jesus to be inferior to the Father.
While the Nicene council paved the way for the homoousian doctrine, there remained many closer to the Arian school who attempted to bypass the Christological debate by saying that Jesus was merely like (homoios in Greek) God the father, without speaking of substance (ousia). These non-Nicenes were frequently labeled as Arians (followers of Arius). Arius objected to Alexander's (the bishop of the time) apparent carelessness in blurring the distinction of nature between the Father and the Son by his emphasis on eternal generation. Alexander accused Arius of denying the divinity of the Son and also of being too 'Jewish' and 'Greek' in his thought. Both Arius and Alexander agreed only on rejecting Gnosticism, Manichaeism and Sabellian formulae. The Nicene Creed was created thus as a result of the extensive adoption of the doctrine of Arius far outside Alexandria, in order to clarify the key tenets of the Christian faith.
Starting on 27 February 380, together with Gratian and Valentinian II, Theodosius issued the decree "Cunctos populos", the so-called "Edict of Thessalonica", recorded in the Codex Theodosianus xvi.1.2. This declared the Nicene Trinitarian Christianity to be the only legitimate imperial religion and the only one entitled to call itself Catholic. Other Christians he described as 'foolish madmen' and terminated official state support for them along with all traditional polytheist religions, practices and customs.
On 26 November 380 Theodosius expelled the non-Nicene bishop, Demophilus of Constantinople, and appointed Meletius patriarch of Antioch, and Gregory of Nazianzus, one of the Cappadocian Fathers as patriarch of Constantinople. In May 381, Theodosius summoned a new ecumenical council at Constantinople to repair the schism between East and West on the basis of Nicene orthodoxy. The council went on to define orthodoxy, including the mysterious Third Person of the Trinity, the Holy Spirit, who, though equal to the Father, 'proceeded' from Him, whereas the Son was 'begotten' of Him. The council also condemned the Apollonarian and Macedonian heresies. From 389–392 the emperor promulgated the ‘Theodosian decrees’, instituting the ascendance of Catholic Church offices, and abolishing the last remaining expressions of prominent non-nicene Christianity with the pagan Roman religions (making the holy days into workdays).