Islam is not simply a revolution brought about by Arabs who, bored of living under their tents, were stirred up by a gifted leader to make a sudden conquest of the most opulent cities of the East. Rather, God allowed the ancient enemy of mankind to have a special opportunity, and to choose an instrument by which he might lead nations astray, enslaving them by the sword. And so there arose Mahomet, the man of Satan, and the Koran, his gospel.
But what was the crime which induced divine justice to go to such an extremity, abandoning nations to a slavery of which we can still see no end? Heresy: for heresy is a dreadful crime which makes the coming of the Son of God into this world to be of no avail. It refuses the word of God; it tramples upon the infallible teaching of the Church. Such a crime must be punished, in order that Christian peoples may learn that no nation resists the revealed words without the danger of suffering, even in this world, the penalty of its rash ingratitude. And so Alexandria fell, though it was Peter’s second see, and Antioch, where he had first been bishop, and Jerusalem, keeper of the glorious Tomb.
The tide was stopped in front of Constantinople, and did not immediately overflow the regions that surrounded it. The Eastern empire, soon to become the Greek empire, was given the opportunity to learn a lesson. Had Byzantium watched over the faith, then Omar would not have come to Alexandria, nor to Antioch, nor to Jerusalem. A delay was granted; it lasted for eight centuries. But when Byzantium had filled up its measure, then the Crescent appeared once more in vengeance. No longer is it the Saracen, who is a spent force, but rather the Turk. Hagia Sophia will see its Christian images whitewashed, with verses from the Koran painted over them. And this is the reason: it had become the sanctuary of schism and of heresy. [. . .]
It dared to penetrate even into the land of France. But a hard expiation it had to do for its boldness, on the plains of Poitou. Islam had made a mistake; where there is no heresy, there it can find no foothold. [. . .]
We shall stop here, having acknowledged the justice of God in regard to heresy, and the true reason of the victories of Islam. We have seen the only reason why God permitted Islam to arise, and why it did not remain an obscure and ephemeral sect in the deserts of Arabia.
We can remember also the words of Leo XIII in Exeunte Iam Anno:
The impartial and unchangeable justice of God metes out reward for good deeds and punishment for sin. But since the life of peoples and nations, as such, does not outlast their world, they necessarily receive the rewards due to their deeds on this earth.