From Kingdom, Grace, Judgment, pages 115-16:

Every now and then at ecumenical gatherings, the Apostles’ Creed is recited; and too often someone gets the kindly meant but misguided idea of substituting the words “holy Christian church” for “holy catholic church.” In terms of the Gospel, that is a disastrous switch. The church is not, in any proper sense, Christian. Its members are indeed called Christians (though it is worth noting that the name was first applied to them, in Acts 11:26 and 26:28, by outsiders); but it is not some sectarian society whose members have a monopoly on the mystery. It is not a club of insiders who, because of their theology, race, color, or sex–or their good behavior, intelligence, or income bracket–are the only channels through which the Word conveys himself to the world. Rather, it is a sign to the world of the mystery by which the Light has already lightened the whole shooting match, by which the divine Leaven has already leavened the whole lump of creation.

Therefore, the church is precisely catholic, not Christian. It is not a sacrament to the few of a salvation that they have but the world does not. Rather, it is the chosen sign of the salvation of the entire world. And (to return to the purchase of the entire field by the man in the parable [of the hidden treasure]) the church has not only to “buy,” or “deal with,” the whole world; it must also, if it is to be any decent kind of sign at all, look as much like the world–and be as little different from the world–as possible.

Yes, I know. The church is indeed to be the salt of an otherwise bland earth. But that doesn’t mean that the church itself is supposed to be all salt or that it is supposed to turn the world into nothing but salt. Therefore, when it represents itself to the world, it probably should not first of all be seen as salt. That’s misleading advertising. You don’t put doughnuts in the window of a shoe store: that only confuses the public about your real business. Likewise you don’t turn the church into a sodality that consists only of bright, white Anglo-Saxons who are happily married, have 1.8 children, and never get drunk. Instead, you just let it be what it in fact already is: a random sampling of the broken, sinful, half-cocked world that God in Christ loves–dampened by the waters of baptism but in no way necessarily turned into perfect peaches by them.