By A.D. 630 all Gaul had long been Catholic.  The last of the
Arian generals and their garrisons in Italy and Spain had become orthodox.
The Arian generals and garrisons of Northern Africa had been conquered by
the orthodox armies of the Emperor.

It was just at this moment, a moment of apparently universal and
permanent Catholicism, that there fell an unexpected blow of overwhelming
magnitude and force. Islam arose – quite suddenly.  It came out of the
desert and overwhelmed half our civilization.

Islam – the teaching of Mohammed – conquered immediately in arms.
Mohammed’s Arabian converts charged into Syria and won there two great
battles, the first upon the Yarmuk to the east of Palestine in the
highlands above the Jordan, the second in Mesopotamia. They went on to
overrun Egypt; they pushed further and further into the heart of our
Christian civilization with all its grandeur of Rome. They established
themselves all over Northern Africa; they raided into Asia Minor, though
they did not establish themselves there as yet. They could even
occasionally threaten Constantinople itself. At last, a long lifetime
after their first victories in Syria, they crossed the Straits of
Gibraltar into Western Europe and began to flood Spain. They even got as
far as the very heart of Northern France, between Poitiers and Tours, less
than a hundred years after their first victories in Syria – in A.D. 732.

They were ultimately thrust back to the Pyrenees, but they
continued to hold all Spain except the mountainous north-western corner.
They held all Roman Africa, including Egypt, and all Syria. They dominated
the whole Mediterranean west and east: held its islands, raided and left
armed settlements even on the shores of Gaul and Italy.  They spread
mightily throughout Hither Asia, overwhelming the Persian realm. They were
an increasing menace to Constantinople. Within a hundred years, a main
part of the Roman world had fallen under the power of this new and strange
force from the Desert.

Such a revolution had never been. No earlier attack had been so
sudden, so violent or so permanently successful. Within a score of years
from the first assault in 634 the Christian Levant had gone: Syria, the
cradle of the Faith, and Egypt with Alexandria, the mighty Christian See.
Within a lifetime half the wealth and nearly half the territory of the
Christian Roman Empire was in the hands of Mohammedan masters and
officials, and the mass of the population was becoming affected more and
more by this new thing.