It is well known (though not as well known–or at least accounted for–as it should be, especially in some conservative Protestant circles) that Augustine had high praise for the Christian emperors Constantine and Theodosius at the end of Book 5 of City of God, which indicates that “political Augustinianism”–whatever that is–does not imply or logically lead to secularism (in the modern, not the Augustinian, sense). Augustine’s qualified praise indicates that he believes that Christian magistrates can be a great good, but that temporal matters should never be mistaken for the highest good or the source of true happiness, and warns that men should never become Christians because they hope to have the earthly success a Constantine, but for the sake of eternal life; if placed in a position of power, they should rule “not through ardent desire of empty glory, but through love of eternal felicity, not neglecting to offer to the true God, who is their God, for their sins, the sacrifices of humility, contrition, and prayer. Such Christian emperors, we say, are happy in the present time by hope, and are destined to be so in the enjoyment of the reality itself, when that which we wait for shall have arrived” (5.24).
Similar advice is found in Athanasius’ Life of Antony 81. As Athanasius presents him, Antony was a man with no interest in politics (and often of a very bizarre piety in certain respects, in my view). When he receives letters from Constantine, Constantius, and Constans, he at first does does not wish to receive them. The emperors are only men, after all, and their writings no more important than those of other men, in contradistinction to what has been written by God in the Law and revealed to us through the Son.1
And the fame of Antony came even unto kings. For Constantine Augustus, and his sons Constantius and Constans the Augusti wrote letters to him, as to a father, and begged an answer from him. But he made nothing very much of the letters, nor did he rejoice at the messages, but was the same as he had been before the Emperors wrote to him. But when they brought him the letters he called the monks and said, ‘Do not be astonished if an emperor writes to us, for he is a man; but rather wonder that God wrote the Law for men and has spoken to us through His own Son.’ And so he was unwilling to receive the letters, saying that he did not know how to write an answer to such things.
After being urged to read them because the emperors were Christians, however, he agrees to receive them. He does not say that Christians should not be emperors. He also does not say that their Christian faith has nothing to do with the discharging of their office. Rather, he keeps first things first: he counsels them “on things pertaining to salvation.” But that, of course, has relevance for how one lives. Christian emperors must “remember that the judgment is coming” and must “know that Christ alone [is] the true and Eternal King.” They must recognize that their “kingship” is only subordinate, that is, and thus in performing it they have obligations to the only “true and Eternal King.” Their judgments should be carried out in light of the coming Judgment, and so “[h]e begged them to be merciful and to give heed to justice and the poor.”
But being urged by the monks because the emperors were Christians, and lest they should take offense on the ground that they had been spurned, he consented that they should be read, and wrote an answer approving them because they worshipped Christ, and giving them counsel on things pertaining to salvation: ‘not to think much of the present, but rather to remember the judgment that is coming, and to know that Christ alone was the true and Eternal King.’ He begged them to be merciful and to give heed to justice and the poor. And they having received the answer rejoiced. Thus he was dear to all, and all desired to consider him as a father.
Antony, like Augustine, does not say that a prince must be a Christian (Augustine certainly doesn’t think so, and his authority in temporal matters is not dependent on his confession, though there are some complicating remarks on this matter in City of God). Rather, he believes that Christians placed in power are under certain obligations and cannot ignore the true order of authority in the cosmos when contemplating how best to exercise their own. There are moral strictures placed upon them; they are to use their position for good and for justice.