The Arabs of the 600s had a similar population explosion and it occurred when the Byzantine empire was weak.
Patricia Crone has pointed out that before the canonical text of the Koran was established, the Caliphs of the Muslim empire called themselves khalifa allaha "God's deputy" and the title did not change until a generation after the canonical text was disseminated into khalifa rasul allaha or "deputy of God's prophet."
In the case of Christianity, the doctrines and history were well-established traditions among believers long before Christianity had any political or military significance. In the case of Islam, the doctrines and history seem to have been elaborated long after the movement had become a powerful force in geopolitics.
Interesting point about the Northmen.
However, the Normans, after their period of rapid expansion, did not turn into a passive people.
In various ways and different degrees Russia, France, the British Empire and the United States are Norman in their origins and character.
The Swedes even had a later episode of being a Great Power for more than a century.
The Arabs have nothing like this. They flared up, died down and never came back to life.
Also the Normans were never anything vaguely resembling a unified state. Their raids and conquests were private ventures, as can be seen in its most extreme form in the conquest of S. Italy and Sicily.
The Arab Empire, OTOH, within about a century had spread as a more or less unified state from China to France.
interesting, thanks...