And so God gave the Romans an Empire, taking away world dominance from the Eastern Empires which had ruled for millennia. In doing so, "He purposely granted it to such men as, for the sake of honor, and praise, and glory, consulted well for their country, in whose glory they sought their own, and whose safety they did not hesitate to prefer to their own, suppressing the desire of wealth and many other vices for this one vice, namely, the love of praise." The Romans, in other words, were not inherently virtuous, but they were at least restrained in their actions out of self-love. This made them "not indeed yet holy, but only less base."
This Roman love of praise is of course not true virtue. "It is, therefore, doubtless far better to resist this desire than to yield to it, for the purer one is from this defilement, the liker is he to God; and, though this vice be not thoroughly eradicated from his heart,—for it does not cease to tempt even the minds of those who are making good progress in virtue,—at any rate, let the desire of glory be surpassed by the love of righteousness, so that, if there be seen anywhere “lying neglected things which are generally discredited,”
if they are good, if they are right, even the love of human praise may blush and yield to the love of truth. For so hostile is this vice to pious faith, if the love of glory be greater in the heart than the fear or love of God, that the Lord said, 'How can ye believe, who look for glory from one another, and do not seek the glory which is from God alone?'"
This glory from God is in the end the only glory which will last forever.
Which is not to say there's no temporal glory--there is, and the Romans have earned it. "They have received their reward."
Christians, on the other hand, have yet to receive their reward: eternal life and felicity ("not a goddess, but a gift of God") in heaven for all eternity. And in this we see the primary purpose for the worldly glory of Rome (the secondary purpose being their own temporal virtue): "And, therefore, it was not only for the sake of recompensing the citizens of Rome that her empire and glory had been so signally extended, but also that the citizens of that eternal city, during their pilgrimage here, might diligently and soberly contemplate these examples, and see what a love they owe to the supernal country on account of life eternal, if the terrestrial country was so much beloved by its citizens on account of human glory."
The point of the city of man is the service of and example to the city of God. We are to look at the city of man and see truths about God on display and worship Him all the more because of it.