Justin I rose from an illiterate Thracian peasant to emperor at nearly seventy, and his chief act was to bring his nephew Justinian to Constantinople.
Justin was born to a peasant family in the Latin-speaking Balkans about 450, walked to Constantinople as a young man to enlist, and rose through the palace guard to command it.
When the emperor Anastasius died in 518 without an heir, Justin was around seventy and commander of the excubitors, the guard closest to the palace. The succession went to him in circumstances the sources describe as a mixture of accident and bribery.
He ruled for nine years, mostly through his nephew, and the reign matters chiefly for what it set up: the religious settlement with Rome, and Justinian.
His origins were genuinely humble in a way the Roman world rarely produced emperors from. Procopius says he arrived in the capital as a boy from Bederiana with nothing but a cloak and a supply of biscuit, which is a literary flourish over a real fact.
He served in the Isaurian and Persian wars, rose steadily on competence, and by 515 commanded the excubitors — a position that put him in the palace at the moment the throne fell vacant.
The sources report that he was illiterate, or nearly so, and used a stencil to trace his signature. Procopius, who hated the whole family, is the loudest witness for this, but it is not implausible for a soldier of his background.
He is one of the harder emperors to see, because he is almost always described in relation to his nephew — either as the vehicle for Justinian's rise or as a simple soldier out of his depth in the palace.
What the record supports is a competent, orthodox, uncomplicated man who had spent fifty years in the army and governed like it: he backed the Chalcedonian party firmly, made peace with Rome, and delegated administration to people who could read.
Procopius's Secret History calls him a fool and worse, but the Secret History calls everyone that, and its portrait of Justin as an amiable idiot manipulated by Justinian should be read as the polemic it is rather than the assessment it pretends to be.
His first major act was religious. The Acacian schism had divided Constantinople from Rome for thirty-five years over the Monophysite question, and Justin ended it in 519 by accepting the papal terms outright.
That decision aligned the empire with the western church and against the Monophysite populations of Egypt and Syria, and the persecution that followed stored up the alienation that made the Arab conquests easier a century later.
Abroad he managed the Persian frontier without a general war, refused the emperor Kavad's request to adopt his son Khosrow — a refusal with long consequences — and cultivated the Christian kingdoms of the Caucasus and Ethiopia. At home Justinian was already running much of the government.
The dynasty he founded ran to 602 and includes the empire's last great reconquest, so his significance is genuine even if it is derivative.
His own contribution was the religious realignment of 519 and the decision to raise Justinian rather than any of the other candidates who might have been advanced.
He is also the last emperor in this archive to rise from the bottom of society through the army alone — a route the later empire, with its entrenched military aristocracy, would make far harder.