Charlemagne impressed his contemporaries as a model king. He was huge of build, full of energy, and enormously successful in almost everything he did. During his reign, Charlemagne built a kingdom that included almost all of western and central Europe and he presided over a cultural and legal revival that came to be known as the Carolingian Renaissance. His empire did not long survive his death. Even before the end of Charlemagne's reign the empire had stopped expanding. Despite its claim as the successor to the Western Roman Empire, Charlemagne's realm lacked many of the important institutions that had allowed the old Roman Empire to survive the emperor's death. Institutions such as a money economy, a strong governmental infrastructure, and a professional civil service were needed to keep the empire from disintegrating. Instead, Charlemagne's empire was based almost entirely on his own personal ability to hold together a large number of different tribes and ethnic groups. The size of the empire made it difficult to administer, and tribal dissension was a frequent threat. The empire collapsed not long after Charlemagne's death in 814. Charlemagne's close alliance with the popes, the leaders of the Roman Catholic Church, also established a precedent for subsequent ties between medieval popes and kings. (Charlemagne Encarta).
Charlemagne's alliance with the Church may have been the reason many people were affiliated with the church. Most people in medieval Europe were very religious. Unlike today, the Catholic Church was the only church in Europe and all Christians belonged to it. With its own laws, lands and taxes, the Catholic Church was a very powerful institution. People knew of other religions and both Judaism and Islam had followers in Europe but all non-Christians were regarded as "infidels" and treated as suspicious. The Catholic Church was based at Rome and headed by the Pope.
Most people believed though that everyone should believe in one religion and the strongest at that time was the Catholic Church, so Christians who opposed the Catholic Church were treated harshly.