Thanks for your input. From what I get there are different strains of two-kingdoms theology. Michael Horton who holds to the view himself said, “Like all great books, his [Augustine] City of God is interpreted rather differently by various schools. However, it is indisputable that it helped to create what came to be called the doctrine of the two kingdoms.” I learned about this matter from YouTube when a preacher called Joel Webbon with right response ministries talked about this. And I agree with his estimation that many today have taken a radical interpretation of two kingdoms to be a distinctive of what is sacred and what is common. Rather, he, Joel, says the way two kingdoms ought to be defined is the way scripture defines it, namely the kingdom of light and the kingdom of darkness. And you don’t find all light in the church and all darkness in the state, but you do find some light and darkness in both those spheres of life. And he would add to that another sphere of life ordained by God, the home.
I've never heard of the language of the two kingdoms used in the way Joel Webbon has. Scripture does speak of our being delivered out of the kingdom of darkness and brought into the kingdom of light as a way to speak of our salvation; that doesn't seem to be at all what St. Augustine had in mind when he wrote the City of God, which was written in response to the sacking of Rome and the fears that many had that the fall of Rome was "the end of the world". Augustine drew a sharp distinction between the temporal and the eternal; between the kingdom(s) of men and the kingdom of God. The fall of Rome means the end of a temporal power in the world, but God's kingdom is entirely other than the earthly power of emperors and kings.
The two kingdoms refers to two forms of divinely instituted authority, as that is what "two kingdoms" refers to. There is temporal authority, which is ordained by God explicitly in this fallen and sinful world for the purpose of the ordering of human society to mitigate the temporal realities and consequences of sin in the context of humans living together. Human beings need social structure, those who exploit and harm the vulnerable and turn their neighbors into victims of grave injustice and evil face the consequences of those actions, and we have laws which say, "Do not do this". However the rule and authority of human government is entirely temporal, it is necessitated because we bear in ourselves the guilt and stain of sin, a brokenness that infects our humanity and by which our natural passions are disordered and we inflict harm against others. Such is the entire institution of human governance and law, secular power is necessitated for the prospering of human life in an unjust world in which we are, ourselves, the agents of injustice against one another.
God's kingdom, that is the heavenly and divine kingdom, is everlasting; and rather than it being about human society and civilization; it is about the fixing and healing of the disordered relationship between man with God. Man, disordered in his passions, estranged from God, is called to repentance, and to hear the good news of God's love and compassion through Christ and His life, death, and resurrection; which is the means of our reconciliation, healing, and restoration. Our sins are forgiven, God declares us righteous on Christ's account.
It is the distinction between our relationship with our neighbors, i.e. Righteousness Coram Mundus, the active righteousness of good works; and our relationship with God, i.e. Righteousness Coram Deo, the passive righteousness, the imputed and received righteousness received through faith.
How we relate toward our neighbor is love and good works, by lives of humility, gentleness, mercy, charity, generosity, clothing the naked, feeding the hungry, giving drink to the thirsty, medical care for the sick, welcoming the foreigner, etc. It is the caring of the widow and the orphan, justice for the poor and the needy, etc.
How we relate toward God is faith in His promises, for He is gracious without measure, who loves us and keeps us and saves us out of His rich and abundant love which He gives us in Christ, who lived, died, and rose again; who reigns at the right hand of the Father with all power and authority, and who comes again at the end of the Age, in judgment, the resurrection of the dead, the healing of the nations, and the renewal and restoration of all things in the Age to Come.
There is what God has commanded, "Do this" because it is good and right, because our neighbor labors, because all of creation labors and suffers. And there is what God promises, "Trust this" because it is what God does and gives, through Christ, which is received through faith out of His uncompromising grace.
So the two kingdoms are another expression of the Law-Gospel Dialectic. This dialectic is, certainly, a deeply Lutheran thing. But it's not limited to the Lutheran tradition, it was embraced (at least technically) by the Reformed as well, and
good Reformed preachers will also know this distinction and of its importance.
-CryptoLutheran