Sphere Sovereignty?

In response to the Donatists, with whom he had a variety of theological disagreements, Augustine developed his 'just war' theory and his understanding of the role of civil government. I found the following quote in one of Augustine's letters to Boniface (Letter 185):

But as to the enactment of those men who are unwilling that their impious deeds should be checked by the enactment of righteous laws, when they say that the apostles never sought such measures from the kings of the earth, they do not consider the different character of that age, and that everything comes in its own season. For what emperor had as yet believed in Christ, so as to serve Him in the cause of piety by enacting laws against impiety...?

How then are kings to serve the Lord with fear, except by preventing and chastising with religious severity all those acts which are done in opposition to the commandments of the Lord? For man serves God in one way in that he is man, in another way that he is also king. In that he is man, he serves Him by living faithfully; but in that he is also king, he serves Him by enforcing with suitable rigor such laws as ordain what is righteous, and punish what is the reverse.