For a long time the Copts and Syrians thought of the Muslims as just another Christian sect. And for a long time the Arabs kept themselves apart from the "infidels." Cairo began as an Arab camp.
Most of the Jews of Palestine welcomed the Muslims as well. About the only people that resisted were the Orthodox Christians of Palestine and Carthage. Things got quite ugly in those places.. The only ones who made any effort to defend Syria were the Byzantine imperial forces, and once they were crushed the way was clear until the Arabs met stiff local resistance from the Orthodox of Anatolia. Egypt was unceremoniously abandoned by the Orthodox after what amounted to little more than a few skirmishes. Of course they planned to return triumphant in short order, but we know how that worked itself out..