Justinian represents, not Roman arms, but Roman law; for not arms but law, and arms only for the sake of law, was Rome’s glory and its gift to the world, the Pax Romana, the boon of world-order and peace.
It may seem a curious irrelevancy that Justinian had first to be corrected in his theology before he was qualified to codify the law, and that the fact is emphasized, in Dante’s way, by the insistent repetition, ‘faith—faith—faith’, to note Justinian’s stages of conviction. For Dante, all earthly order comes by divine ordinance. Just law belongs to the context of the Gospel. The Empire is the Empire of God’s intention only when it is in harmony with the Church, both Church and Empire respecting with a mutual loyalty the other’s province. So the Emperor was instructed by the Chief Shepherd in the true faith—the faith that Christ was human as well as divine, and therefore concerned with the earthly order—, and it was only when the Emperor ‘took his way beside the Church’ that he was inspired to his ‘high task’. So Augustus made peace on earth for the birth of Christ; so Tiberius and Titus were the authorized instruments of divine judgement in the Crucifixion and in the destruction of Jerusalem; and so Charlemagne delivered the Church from the Lombard tooth.