From Central Asia, the Huns started the ethic flow to the west. No other ethic group seemed to match their martial skills and ferocity. They are credited with annihilating everything in their path! By 370 AD, the Goths had been divided by the Danube with a western branch (Visigoths) and an eastern branch (Ostrogoth). Their western flow pressed on to the City of Rome itself, being sacked by Alaric, "The Visigoth" in 410 AD.
It was at this time that the Christian Church was trying to make heads or tails out of all these happenings. A mystic in North Africa named Augustine, Bishop of Hippo, wrote down his thoughts. It certainly contained the view held by what was left of the Church at Rome. [Augustine began to write his book three years after Alaric completed his work in the city of Rome.] The destruction of the city of Rome, who many had thought "would stand forever", left just as many feeling demoralized. There were still folks remaining in the Empire who wanted to blame these "Christians" for its very downfall. Of course the Christians wanted to blame the "pagans". Who indeed were guilty for all this catastrophe.
In 22 chapters (called Books), Augustine writes his "City of God". In Book I, he censures the pagans especially for the sack of Rome. Book II - Book III he reviews the calamities suffered by the Romans before Christianity, claiming their gods had plunged them into corruption and vice. He then presents his view of theology, and the workings of a "Supreme God" who really had things in hand all along. [Book IV- Book XXII] The "real" city was not Rome, but a "City of God". In his first paragraph Augustine writes:
"For the King and Founder of this city of which we speak, has in Scripture uttered to His people a dictum of the divine law in these words: 'God resisteth the proud, but giveth grace unto the humble.'"
The flow and ebb continues...right up to the Celtic Church located at the end of the world.
Reference: "The City of God by Saint Augustine", translated by Marcus Dods, with an Introduction by Thomas Merton. The Modern Library, NY. Random House, Inc. 1950.