One of the things that bothers me about Bonhoeffer's Sanctorum Communio is his thesis that when we are talking about the church we are talking about 'Christ existing as community'. This leads him to say things like 'Christ... is present only in the church'. This makes me pretty uncomfortable.
I suspect, although it would take a good deal more reading to prove it, that this a peculiarly Lutheran formulation. Bonhoeffer does not mean that wherever a group of people gather together and decide to call themselves a church, there we have Christ. At the heart of his conception is Word and sacrament. Like a good Lutheran, for Bonhoeffer wherever the Word is preached and Holy Communion is administered, there is Christ - and therefore the community gathered around these things not only gathers around Christ, but actually is Christ: Christ existing as church-community.
I think this goes wrong theologically at two points. Firstly, in terms of Christology, I think it is important to stress that although Christ is certainly present to and in his church in a way which he is not elsewhere, he is nevertheless a distinct person from his church. Christ is one; the church is another. No amount of emphasis on the spiritual union of Christ and the church (which I think is best described by reference to the Spirit, as below) overcomes this absolute distinction. Moreover, despite this special presence in and to the church, Christ is in fact ascended - his body is in heaven - and exists there and not here, I think it is hugely important that we say that the incarnation is one thing, and the presence of Christ with the church is another; the latter is not an extension of the former, because the incarnation doesn't need extending - Christ is still incarnate in glory.
Secondly, in terms of Pneumatology, I think the view which I am (perhaps mistakenly) attributing to Bonhoeffer does not take seriously enough the role of the Holy Spirit in making the Word and sacrament efficacious. If Christ is present in and to his church, it is primarily by his Spirit, who ministers the Word and sacrament to us. Christ is not bound to the ministry of his human servants in the church, but comes by the Spirit. Whilst we look to the Word and to the sacrament as means by which Christ has promised to come, we must acknowledge that everything depends on the Spirit - and I would argue that also means we must recognise the possibility that the Spirit will move outside the church.
There is a practical problem here, as well. It seems to me that Bonhoeffer's view is just one way of over-selling the church - making it more than it really is. I think this view more or less divinises the church, I sometimes think we do this in lots of different ways - make the community more important than it is. Perhaps it's the pendulum thing again - in seeking to avoid the individualism of our culture, we've swung too far the other way. This can be quite subtle. It could be something as simple as language - for example, using the language of 'incarnation' to describe the church, giving the impression (even if this is not intended) that we think of Pentecost as sort of like a multi-site Christmas. Or it might just be when we hold up the church community as evidence for the truth of the gospel - God must be at work, look at how much we love one another! - as if the real work weren't done and finished 2000 years ago. Or just talking as if the very community life of the church was in itself transformative for the world - as if just getting people to taste church community would turn them into Christians.
I'd hate to be mis-read here. Church is vital. To become a Christian is to be united to Christ by his Spirit and therefore to be united to his community. Local congregations of Christian believers are the primary means by which Christ nurtures his people and reaches the world.
But each congregation is human - all too human. Just a bunch of people, having perhaps more in common than we would like to admit with any group of hobbyists or social club. I worry that we set people up for disappointment by our overly effusive praise of church. I worry that their disappointment will spill over into disappointment with God. If we gather expectantly around the Word, well then we have something to show to the world - but it isn't us in any way. It's Christ.