Question

A period of lawlessness and disunity followed the fall of the Roman Empire. What served as the only force in Europe

Answer 1

The remaining force after the fall of Rome was Christianity.
After the fall, the former western empire saw the rise of monasteries which provided people with religious instruction and some form of unity and renewal. The power of the papacy became increasingly strengthened from then,through the dark ages to medieval times.

Answer 2

The only unifying force in Europe following the fall of the Roman Empire was the Christian Church (Christianity). Emperor Constantine issued the Edict of Milan in 313 AD accepting Christianity. It became the official religion of the Roman Empire ten years later.