from The Synaxarion:
Saint Justinian, a major figure in the history of the Byzantine state, was also a great champion of Orthodoxy, a builder of churches and a Church writer, and he was of Slavic descent -- born in Bulgaria. During his reign (527-565) Byzantium won glory with military victories in Persia, Africa, Italy, -- as a result of which paganism was decisively rooted amongst the Germanic Vandal and West-Goth tribes. By command of the emperor Justinian the pagan schools in Athens were closed. With the aim of spreading Christianity through the regions of Asia Minor, Justinian sent there the bishop of Ephesus John, who baptised more than 70 thousand pagans. The emperor gave orders to build 90 churches for the newly-converted, and he generously supported church construction within the empire. His finest structures of the time are considered to be the monastery at Sinai, and the church of Saint Sophia at Constantinople. Under Saint Justinian many a church was built in the name of our Most Holy Lady Mother of God. Being a man of quite diverse an education, Saint Justinian assiduously concerned himself over the education of clergy and monks, ordering them to be instructed in rhetoric, in philosophy and in theology.
The right-believing sovereign devoted much attention and effort into the struggle with the Origenists of his time, who then were reviving the Nestorian heresy. Against their heretical speculations was composed the Church-hymn "Only-Begotten Son and Immortal Word of God, Who for our salvation...", and he commanded its singing as obligatory in the churches. From that time through the present day this hymn is sung in the Divine Liturgy before the Small Entrance [i.e. 2nd Antiphon]. At the command of the sovereign, in the year 553 was convened the Fifth Ecumenical Council, censuring the teachings of Origen and affirming the definitions of the Fourth Ecumenical Council at Chalcedon. The holy Emperor Justinian about orderly rule and law within the realm. Under his guidance and supervision was compiled a complete compendium of Roman laws, which has come down to us as a codex of law known as "the Justinian Codex". The "Novellae" (i.e. "Church-laws") of Justinian find inclusion in all the variants of the Russian Church-law Nomo-Kanon Books.
In his personal life, Saint Justinian was strictly pious, and he zealously fasted quite often. The holy Emperor Justinian died in the year 565.