In A Call to Spiritual Reformation, D.A. Carson writes,
The Church is to see itself as an outpost of heaven. It is a microcosm of the new heaven and the new earth, brought back, as it were, into our temporal sphere. We are still contaminated by failures, sin, relapses, rebellion, self-centeredness; we are not yet what we ought to be. But by the grace of God, we are not what we were. For as long as we are left here, we are to struggle against sin, and anticipate, so far as we are able, what it will be like to live in the untarnished bliss of perfect righteousness. We are to live with a view to the day of Christ.Unfortunately, many people look at the church and see only its "contaminations." The church is often accused of being full of hypocritical, self-righteous, unloving people. Then disillusionment and resentment begin to set in. Yet we must not neglect to see God in the process of redeeming His people as a chosen race, a royal priesthood, and a holy nation (1 Peter 2:9). Yes, the church has many blemishes (because it's comprised of sinful people), but it is also a testimony of God's grace, as He purifies and prepares us for that glorious day when the Bride of Christ will be presented to Jesus Christ, the Bridegroom. And until that time, the church must remember our mission as an "outpost of heaven."
That means, of course, that Christians constitute a kind of missionary community...until the consummation, we live out our lives down here, a heavenly, missionary outpost in a lost, dying, and decaying world. We are to see ourselves as an outpost of a new heaven and a new earth in an old world that stands under the judgment of God.