The introduction of images into the Christian Church had confused the simplicity of religious worship.  The education of Leo, his reason, perhaps his intercourse with Jews and Arabs, had inspired him with a hatred of images.  By two edicts he proscribed the existence, as well as the use, of religious pictures.  This heresy of Leo and of his successors and descendants, Constantine V. (741), Leo IV. (775), and Constantine VI. (780), whose blinding by his mother Irene is one of the most tragic stories of Roman history, justified the popes in rebelling against the authority of the emperor, and in restoring and establishing the supremacy of Rome.

Gregory II. saved the city from the attacks of the Lombards, who had seized Ravenna and extinguished the series of Greek exarchs in 751.  He secured the assistance of Pepin, and the real governor of the French monarchy—­Charles Martel, who, by his signal victory over the Saracens, had saved Europe from the Mohammedan yoke.  Twice—­in 754 and 756—­Pepin marched to the relief of the city.  His son Charlemagne, in 774, seemed to secure the permanent safety of the ancient capital by the conquest of Lombardy, and for twenty-six years he ruled the Romans as his subjects.  The people swore allegiance to his person and his family, and the elections of the popes were examined and authorised by him.  The senate exercised its rights by proclaiming him patrician and of the power of the emperor; nothing was lacking except the title.

A document, known as the Forged Decretals, which assigned the free and perpetual sovereignty of Rome, Italy, and the provinces of the West to the popes by Constantine, was presented by Pope Hadrian I. to Charlemagne.  This document served to absolve the popes from their debt of gratitude to the French monarch, and excused the revolt of Rome from the authority of the eastern empire.

Though Constantinople returned, under Irene, to the employment of images, and the seventh general council of Nicaea, September 24, 787, pronounced the worship of the Greeks as agreeable to scripture and reason, the division between the East and the West could not be avoided.  The pope was driven to revive the western empire in order to secure the gift of the exarchy, to eradicate the claims of the Greeks, and to restore the majesty of Rome from the debasement of a provincial town.  The emperors of the West would receive their crown from the successor of St. Peter, and the Roman Church would require a zealous and respectable advocate.

Inspired by these motives, Pope Leo, who had nearly fallen a victim to a conspiracy (788), and had been saved and reinstated by Charlemagne, took the opportunity presented by the French king’s visit to Rome to crown him emperor.  On the festival of Christmas (800), in the church of St. Peter, Leo, after the celebration of the Holy Mysteries, suddenly placed a precious crown on his head.  The dome resounded with the acclamations of the people, his head and body were consecrated with the royal unction, and he was saluted, or adored, by the pontiff after the example of the Caesars.