I give you a brief sample from Against Christianity. Is he here challenging the fundamental nature of "Christianity" or simply our Western conception that individuals constitute societies? If we instead conceive that this is backwards—that societies in fact constitute individuals—are Leithart's words still so shocking?
Ask the average Christian about the relationship between "church" and "salvation," and you are likely to get one of two answers: either (if the Christian is a rather old-fashioned Roman Catholic) that the Church is the reservoir of salvation, to which one must repair to receive grace; or (if the Christian is a rather common sort of evangelical) that salvation occurs apart from the Church, though it is a help along the way.
Despite the apparent differences between these two views they are fundamentally similar. Both conceive of "salvation" as a something (almost a substance) that can be stored in a reservoir or infused into sinners directly by God. Both believe that the whole point is the salvation of individuals: for the Catholic, the Church is an essential conduit of grace: but salvation is what happens to the individual; for the evangelical, the Church is a nonessential aid to individual salvation. In both cases, Christianity is looming in the background.
Biblically, however, salvation is not a stuff that one can get, whether through the Church, or through some other means. It is not an ether floating in the air, nor a "thing," nor some kind of "substance." "Salvation" describes fallen creation reconciled to God, restored to its created purpose, and set on a trajectory leading to its eschatological fulfillment. Ultimately, "salvation" will describe the creation as a whole, once it is restored to God and glorified. Grammatically, "salvation" is a noun; theologically, it is always adjectival.
Nor is salvation adjectival merely of individuals. If salvation is the re-creation of man through Christ and the Spirit, then salvation must be restored relationships and communities as much as individuals. If Christ has not restored human community, if society is not "saved" as much as the individual, then Christ has not restored man as he really is. Salvation must take a social form, and the Church is that social form of salvation, the community that already (though imperfectly) has become the human race as God created it to be, the human race that is becoming what God intends it to be.
The Church is neither a reservoir of grace nor an external support for the Christian life. The Church is salvation.