Oh, and the thing about Muhammad and his impact on the course of history is that his mobilization of the Arabs had to happen right when it happened in order to have the profound subsequent effects that it did. It was right when he enflamed them with the sword of Islam that the Byzantine and Sassanid empires were at their lowest ebb, thereby permitting the Muslim conquest. Even twenty years later would've almost certainly been too late, and almost certainly having been contained at the start, the upstart creed would've turned inwards to become an almost purely ethnic Arab cult.
Yes, I think that generally speaking religions evolve to match the prevailing ethos, but there are certainly in my view events in history that are extraordinary departures from the otherwise course of human affairs. Islam was not merely a religious phenomenon, as Hart points out, but just as much a political phenomenon, and in order for the political phenomenon to play itself out it had to be right then and there.
Why would 20 years later have been too late in your view?