Christopher Manion tells us why some find the gospel message (and Mel Gibson's upcoming movie) offensive offensive:
"The plain truth, of the Gospels and of the controversy that has surrounded Christianity ever since Christ’s death and resurrection, is simple. If a person does not believe the Gospel to be the word of God, the only alternative is to find it offensive. Very offensive. Fundamentally offensive. And for good reason.
It is offensive."
And that's it in a nutshell.