If you start your investigation of human nature with Jesus - if Christology is at the centre of your anthropology - the possibility is raised that you do not know yourself. You may have to be told who you are. It may be that none of us knows inherently, or is able to deduce from our own behaviour or that of others, what it means to be human. That raises all kinds of issues.
One issue that has been floating around recently, mainly because of the case of Mr and Mrs Johns, is the attitude of Christians to homosexuality. Now, up front, I want to acknowledge that this is a hugely complex and, for many, painful issue. I also want to acknowledge that Christians don't all agree on this topic. But I have to say that as I read Scripture, and as I try to think theologically about what the gospel implies for humanity, I arrive at the conclusion that an active homosexual lifestyle is not compatible with the Christian message. I don't want to overplay that, and I also don't want to make out that this is a big part of my belief - it is not. (In fact, although from the news you would get the impression that evangelical Christians basically talk about this all the time, and also that they are raging homophobes, my experience has been that the topic comes up rarely, and when it does is tackled with pastoral sensitivity. I know that hasn't been everyone's experience.) Still, however peripheral this belief is - however much I may consider it to be basically an implication of an implication of the gospel, something which stands at considerable remove from the heart of the faith - nevertheless I am obliged to hold it.
And here's the point. This position comes in for criticism so often in the news, and raises such outrage amongst our contemporaries, because it challenges our notion of what it means to be human. Christians are saying something which is, despite all my disclaimers above, huge in what it means for human nature. Everybody knows that Christians want to tell people how humanity ought to be - it is part of the Christian message to say that we are not what we should be, and that we must be changed. This is, naturally, offensive to people. But I think we are actually saying something more offensive than that - we are saying that you are not who you think you are. Imagine if a Christian said this: 'if this lifestyle is incompatible with the gospel, it is not human'. Offensive! But that is implicitly what we are saying.
Of course, we are saying it to everyone, not just those who practice a certain lifestyle, and not just those outside the church. Still, sexuality is a point at which it is particularly painful, because it is so close to the heart of an individual's identity. The Christian message threatens to take their identity away, by telling them they are not really who they think they are at all. What insecurity this threatens! And what offence!
The flip-side is that if the gospel is true, then I am told who I really am, and can relax. I have no need to forge my own identity, assert my own individuality, or even wrestle with my own inner contradictions to quite the same extent. Who I am is decided elsewhere, and I am in a sense graciously given to myself to enjoy.