Too often we tend to talk about the church as the body of Christ in a way that occludes the distinctly Christological and soteriological importance of this biblical image. The way the image tends to function in much theological discourse is to append Christ to the church in such a way as to bolster the church’s own institutional self-confidence and certainty. It simply functions to assure us that the church is in continuity with Christ and is therefore in the right.
But as Bonhoeffer points out beautifully in Ethics, the body of Christ language in Scripture serves first and foremost to point us to Christ and his act for the salvation of all humanity in the cross and resurrection:
Above all we must turn our eyes to the image of Jesus Christ’s own body — the one who became human, was crucified, and is risen. In the body of Jesus Christ, God is united with humankind, all humanity is accepted by God, and the world is reconciled to God. In the body of Jesus Christ, God took on the sin of all the world and bore it. There is no part of the world, no matter how lost, no matter how godless, that has not been accepted by God in Jesus Christ and reconciled to God. Whoever perceives the body of Jesus Christ in faith can no longer speak of the world as if it were lost, as if it were separated from God; they can no longer separate themselves in clerical pride from the world. The world belongs to Christ, and only in Christ is the world what it is. It needs, therefore, nothing less than Christ himself. Everything would be spoiled if we were to reserve Christ for the church while granting the world only some law, Christian though it may be. Christ died for the world, and Christ is Christ only in the midst of the world. It is nothing but unbelief to give the world — for well intentioned pedagogical reasons to be sure, which nonetheless leave an aftertaste of clericalism — less than Christ. It means not taking seriously the incarnation, the crucifixion, and the bodily resurrection. It means denying the body of Christ. (pp. 66-67)
Bonhoeffer goes show how understanding the church as the body of Christ means not that “the church-community is first and foremost set apart from the world. On the contrary, in line with the New Testament statements about God becoming flesh in Christ, it expresses just this — that in the body of Christ all humanity is accepted, included, and borne, and the the church-community of believers is to make this know to the world by word and life” (p. 67).
Thus the church is understood as the body of Christ first and foremost in terms of soteriology and Christology. In Christ’s incarnation, death, and resurrection God has taken on the flesh of all humanity (or rather all people participate in Christ’s own distinct humanity) and the church is the proleptic sign and sacrament of this reality. Thus, the church as the body of Christ is neither a metaphor or something to be explained in mystical terms. Rather it is a Christological reality. The church is the body of Christ in that it is the sign and sacrament of the event of Christ in which all human flesh, indeed the whole world is united and transfigured in the love of the triune God.