The biggest injury to that “Great Leap Forward” towards an open, egalitarian, universalist society (to borrow Mao’s phrase and put it to much better use) was actually the shattering of Christendom. This came via the triple hammer blows of (1) the Black Death and the tremendous socio-economic upheaval it produced [now there was the pandemic of pandemics!], (2) the Reformation followed by the Wars of Religion, and (3) the Enlightenment, which, despite all its protestations to be the Age of the coming of the great light of emancipation from superstition, opened the doors wide to the tsunami called the French Revolution.
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.