" Arabs had always been until then a marginal border people, used as mercenaries at best, but no meaningful threat – there was hardly even an armed defence on the largely desert Arabian frontier. They could hope that it would be reversed, but when the first Arab civil war of 656–61 did not lead to the break-up of the coherence of the new caliphate, and Arab raiding into Anatolia increased instead, it became clearer that the new political order was here to stay. The Romans did not understand what Islam was yet – it was initially seen as a simplified form of Christianity, not a new religion – but, either way, given the way east Roman political imagery now worked, this was as much a religious catastrophe as a military one, since the victorious Arabs were certainly not Orthodox Christians. "

― Chris Wickham , Medieval Europe