The darkest hour of the State was the darkest hour of the Church. The last faint traces of the Carolingian revival of religion disappeared amid the horrors of Danish, Saracen, and Hungarian invasions. The feudalism that saved Europe from the barbarians now began to infect what remained of Christian life with its own ferocity, greed, and lust. The spiritual offices of the Church were becoming heritable property, dissociated from all effective spiritual duties. But amidst the turmoil of feudal times, a few nobler spirits sought salvation from the wickedness that lay thick around them in the solitude of the cloister. Before the end of the tenth century, the Cluniac revival presented to Europe an ideal of life very different from feudal militarism. In alliance with the Empire, the Cluniacs restored religion in central Europe, and missionaries, working in their spirit, spread the Gospel beyond the bounds of the Empire among the barbarians of the north and east. But from Cluny also came new theories of the province of the Church, which soon brought religion into sharp conflict with the temporal authority. When the power of the State lay almost in abeyance, it was natural that the Church should encroach upon the sphere it left vacant. From Cluny came the Hildebrandine Reformation, and from the theories of Hildebrand sprang two centuries of conflict between Papacy and Empire. [Sidenote: The struggles of Papacy and Empire.] The great struggle of Popes and Emperors (the highest expression of the universal struggle of the spiritual and temporal swords), was the central event of the Middle Ages. It first took the form of the Investiture Contest, but when the Investiture Contest had been ended by the substantial victory of the Church, the eternal strife was soon renewed under other pretexts. It inspired the contest of Alexander III. with Frederick Barbarossa, of Thomas of Canterbury with Henry of Anjou, of Innocent III. with half the princes of Europe, and the final great conflict between the successors of Innocent and Frederick II. At last the Empire succumbed before the superior strength of the Papacy. But the Hohenstaufen were soon revenged; and, within two generations of the death of Frederick II., the victorious Papacy was degraded from its pride of place by its ancient ally. [Sidenote: Religious and monastic revivals.]