This message called The Supremacy of God and the Gospel in a Post-modern world is an important and helpful one. We are living in a different world- the trouble is most of us have not realised it. An ignorance of the gospel of grace and its contextualisation is having its impact. Yes, some things like Alpha are effective (it gets a mention from Keller) but even it is not doing particularly well when it comes to engaging the un-churched.
If you are connected in any way with thinking about the gospel in our days get a pen and paper and take a few notes on this one. It's one to listen to twice.
By the way, Keller's new book called The Freedom of Self-forgetfulness