Perils from without assail the Catholic Church openly throughout the centuries. Perils from within threaten more insidiously.

At the close of the Middle Ages the invading armies of Islam threatened continually the faith of Christendom, and the good estate of the Holy See; while within the seeds of disaffection, neglected because unseen, ignored so blandly and when visible so virulently denounced, germinated in every land of Europe north of the Alps and Pyrennees. The Mohammedan hosts were driven back. Rome was sacked by the Lutheran troops of the Catholic emperor, the success of Protestantism accomplished the break-up of the Catholic unity of Christendom.

The peril from without, quite a real peril, failed to overthrow Christendom, and in Europe (though hardly in Asia, and certainly not in Africa) the faith of Islam has steadily diminished. But Protestantism, the peril from within (for the men who made the Protestant Reformation were all Catholic priests or Catholic laymen), survives throughout the world, and is for ever threatening the faith and morals of Christian people.