[I]t may be instructive to examine this "brusque" utterance of the Emperor and ask the question: Is it simply indefensible?
As a thought experiment, let's reverse the situation. Suppose a major spokesman for Islam publicly issued the challenge: "Show me just what Jesus brought that was new, and there you will find things only evil and inhuman." What would be the Christian response? Not to burn a mosque or an effigy of the Muslim spokesman, or to shoot a Muslim nurse in the back in Somalia. It would rather be to reply with some examples of just what makes the New Covenant new: the revelation that God is a Father who has a co-equal Son and Holy Spirit; that Jesus is God's Son made flesh; the Sermon on the Mount; the Resurrection of the body; the list would be long. As Irenaeus put it: he brought all newness, bringing himself. Such a statement would not make dialogue impossible; it would be an occasion for dialogue.
Yup. Is the reasoning behind Islam so embarrassingly awful that it can't forthrightly answer a simple question?