Lactantius was a Christian who had been a part of the Roman government (which was hostile to Christians) prior to his conversion. Sometime between 303 and 311 he wrote a systematic theology, in which he wrote:
When [God] might have bestowed upon His people both riches and kingdoms, as He had before given them to the Jews, whose successors and posterity we are; on this account He would have them [that is, "His people"] live under the power and government of others, lest, being corrupted by the happiness of prosperity, they should glide into luxury and despise the precepts of God; as those ancestors of ours, who, ofttimes enervated by these earthly and frail goods, departed from discipline and burst the bonds of the law.
But when Constantine converted to Christianity and became emperor, Lactantius became an advisor and hagiographer to Constantine, as well as a tutor to the emperor's son. It appears that he believed it good and right for a Christian to hold the highest post in the government, even though he had previously said that it was for Christians to live under a pagan emperor.
Did he ever say why it was good for Christians to live under a Christian government?