Some of my friends thought that Against Christianity was a tad too Anabaptist. At first I had trouble seeing this, but the more I studied the differences between Reformed and Anabaptist, particularly through reading Yoder’s Anabaptism and Reformation in Switzerland, I began to see where those critiques were coming from. The first generation of Anabaptists really meant that Christians were of the new polis and not the old one.
But happily, Leithart is not to be underestimated. He has recently given some thoughts on the qualified nature of Church as polis, which can be found here. A few lines are worth pointing out:
On this view, the church is never absorbed into earthly political order, never reduced to a national cult, even if the nation is thoroughly Christian. The political order to which the church belongs is the eschatological political order of the heavenly city. At the same time, this view seems to avoid some of the separationist tendencies that sometimes infect church-as-polis theories. The church isn’t defined over-against the earthly city, but as the sacrament and “cult” of the city that is to come.
And just for the record, this is also what the Bible thinks. 
There would have to be two separate courts (church and state), though, right? That would seem to be required by the fact that the state should not coerce belief (while the church should, though not with literal violence). I’m a little uneasy with the idea that a Christian state would take over all church discipline.
Not that this changes the basic point that the church is not a polis in the full sense until the eschaton. Leithart’s point would seem to be consistent with Hebrews, too, which talks about us “coming to Zion” in worship, implying we’re not always Zion in our day to day lives (or at least not in the same sense, etc.).
Good post!
I suppose too that preaching the Word itself would be a form of discipline (in fact, I’m sure of it, with the Word of God being a sword dividing marrow, and being useful for correction, etc.), the declaration of God’s judgments. And the sacraments as well, since they bring us into the presence of Christ to be judged/blessed.
But again, none of this contradicts your point.
(sorry for filling up the com-box, but I’m on a roll…)
And, even though we may not want to make one form of government a mark of the church, I think it would be hard to argue that government of some form is not necessary for a church to be a church. Jesus commanded communal self-discipline (Matt 18:15ff) just as much as he commanded baptism and the Lord’s supper… not sure by what logic we could conclude that the first is not essential while the later two are.
So perhaps it would be permissible, in the event that the state becomes Christian, for the state to do some of the things the church might have done in a minority position (punish theft, for example), but I don’t think that would imply that a church could be not “en-polis-ed” (that is, having some sort of corporate self-discipline) and still be a church.
Grist for the mill.
(one more time!!)
Also: unless you think the state should force people to participate in the sacraments/worship, there would by necessity be a church court to deal with dereliction in those things.