Charlemagne’s dream was certainly more noble than Constantine’s, and the new Emperor of the West seems to have had a very sincere faith in Christ and a desire to see it established and inculcated into the hearts, minds, and culture of the peoples under his sway. He promoted learning and study and extensively built churches, monasteries, convents, schools, hospitals, and castles for his garrisons. He was devout in his personal observance. But he still used fear and force to convert the reluctant or make examples of the too stubborn.
Part of Rome’s genius was adoption and adaptation—the ability to absorb and assimilate all comers, repurposing them to serve Rome’s dominant vision as the great civilizer of the world, the great unifier giving everyone equal access to the same gods and guiding principles. The Emperor was the supreme symbol, the creator and maintainer of this unity—the “Saviour of mankind”, the “Son of God” (Jupiter, Zeus, Amon-Ra, Baal, whichever high deity was relevant to the people in question). Every subject and citizen of the Empire owed their final allegiance to the Emperor as the incarnation of Rome’s “genius”, or “Spirit-Guide”.
Rome incarnated a direct claim by humans to establish an eternal kingdom on earth by right of conquest and coercive power. Local gods could bow and be absorbed into Rome’s in order to survive, or be annihilated like those of the Carthaginians and Druidic Celts. The Jews and Christians challenged Rome’s nature at its root. Both paid a massive price in millions of lives for continuing to seek and honour the true Creator.