It sprang from a lot of things - such as the power void in the area after the collapse of the Roman Empire, the weakness of the Church as a unifying force after the Arian and other heresies, etc. But Johnson is absolutely right about the fact that the feeble theological response was because of weak or confused thought on the Trinity and the two natures. Mad Mo thought he had the winning combination by dumping them both and simply creating a weird synthesis of easy-to-understand monotheism, bits of pagan cult and practices, and a touch of Jewish ritual law. And of course, that thing about the armed militias, which were his special personal contribution to the mix, being of bandit stock himself.
In any case, I think Johnson has some inaccuracies or generalizations that are a little too broad; the behavior of the Church has certainly never been beyond reproach, but it has never hit the profound doctrinal level. Also, don't forget that there were times when other Catholic groups (the French and the Spanish, specifically) sacked Rome and attacked the Pope over purely political things. But still, this had nothing to do with Christian doctrine.
On the other hand, with Islam, it has everything to do with doctrine, something of which the Pope (alone) seems to be clearly aware.
BTW, a little off-topic, is it only me, or is anybody else here really terrified about what would happen to women if Islam took over? When I wrote to the Pope, I mentioned that as one of my main concerns.
The Pope also said that there exists Islamic doctrine that eschews violence, didn't he?
I don't mean any offense by this, but I'm going to have to take Johnson's view over yours, as you should well take his or any historian of your choosing, over my mine, as Johnson had access to documents and info that you and I don't. Assuming of course, that you are not a historian of his caliber and CV.
What seemed obvious to me, and the point I was trying to get at was that once this mutual disclaimer is out of the way, a better climate is created for 'dialogue.' Though it is doubtful, absent the emergence of a Islamic Churchill or Bonhoeffer and the likes of the Churchmen that surrounded him, it will still come to naught.
And as far as your comment on women and Islam, to me that's where their disease lies. They absolutely hate women, and I don't quite understand why? A couple of years ago there was an article written about the stoning of an 'adulteress' that really made me nauseous.