It was by the Will of God that Constantine became possessed of the Empire
Thus then the God of all, the Supreme Governor of the whole universe, by his own will appointed Constantine, the descendant of so renowned a parent, to be prince and sovereign: so that, while others have been raised to this distinction by the election of their fellow-men, he is the only one to whose elevation no mortal may boast of having contributed.
Source: Life of Constantine (New Advent)