Mohammedanism, shortly after its birth (622), began to threaten Christianity. After having driven the Christian Church from northern Africa, the followers of Islam overthrew the Visigothic power in Spain (711) and then swarmed across the Pyrenees to overrun most of France. The very existence of Christendom was at stake, and the future of Europe hung in the scales and might have been very different, had not Charles Martel with his stalwart Christian knights in the bloody battle of Tours (732) checked the advance of the crescent and forced its adherents to hastily retrace their steps. The califate founded at Cordova (756) continued as a standing menace for more than six centuries. Meanwhile Moslem corsairs scoured the Mediterranean, seized Sicily, and from that vantage point sought to make a conquest of Italy venturing at times to the very gates of Rome. The contest between the faithful of these two religions, continued for centuries and attained its climax in the crusades. The followers of each faith sought to either conquer or exterminate the other. This form of missionary work was like that employed by Charles the Great against the Saxons and Otto the Great against the Slavs. The repeated assaults of Frankish rulers, Spanish princes, and Norman warriors in Italy were finally successful and Islam was thrust back into Africa, but only to enter Europe by way of Constantinople. In sharp contrast to these harsh methods, there are not a few instances of devout Christians labouring in love among the followers of the Prophet to save their souls. Conversions to Christianity were not infrequent in Spain, Italy, Egypt, and the East.[259:1] The Franciscans and Dominicans both laboured heroically among the followers of the Prophet to teach them the higher and better faith.[259:2] Notwithstanding the fact that Christianity spread so rapidly throughout the Roman Empire, yet it must be remembered that more than twelve centuries were to circle away before the cross was carried to all European peoples and planted among them. The problem was as difficult as that encountered to-day in Africa, Asia, and the islands of the seas. By the twelfth century all Europe, except Lapland and Lithuania had been won to Christianity. If the number of Christians approximated 30,000,000 at the death of Constantine, the number at the time of Pope Innocent III. in 1200 may have been 200,000,000 who came within the direct or indirect jurisdiction of the Christian Church. The sweeping control of the Roman Church gathered under her broad ægis possibly 100,200,000. Through these missionary activities, therefore, the successor of St. Peter had extended his actual sway until it included all of western and central Europe with a population as large as that of the Empire of Cæsar at the birth of Christ.