II. The extension of the Roman empire was part of the Divine scheme
For the good, just, and Almighty God, Who has never withheld His mercy from mankind, and has ever instructed all men alike in the knowledge of Himself by the most abundant benefits, has by a more secret counsel and a deeper love shown pity upon the wanderers' voluntary blindness and proclivities to evil, by sending His co-equal and co-eternal Word. Which becoming flesh so united the Divine Nature with the human that He by lowering His Nature to the uttermost has raised our nature to the highest.
But that the result of this unspeakable Grace might be spread abroad throughout the world, God's Providence made ready the Roman empire, whose growth has reached such limits that the whole multitude of nations are brought into close connection. For the Divinely-planned work particularly required that many kingdoms should be leagued together under one empire, so that the preaching of the world might quickly reach to all people, when they were held beneath the rule of one state.
And yet that state, in ignorance of the Author of its aggrandisement though it rule almost all nations, was enthralled by the errors of them all, and seemed to itself to have fostered religion greatly, because it rejected no falsehood. And hence its emancipation through Christ was the more wondrous that it had been so fast bound by Satan.
Source: Sermons (New Advent)