Christianity has its roots in the Pharisaic tradition, and early followers of Jesus saw themselves as Jews for whom Jesus’s life served as a model. The key to Paul’s faith was not Jesus’s life, although that was a model for Christian ethics, but Jesus’s death and resurrection. He also began to convert Gentiles to Christianity. Early Christianity frightened conservative Romans because Christians met in small groups, had secret rituals, and refused to make patriotic sacrifices to the emperor cults. Although Constantine was responsible for the acceptance of Christianity in the Roman Empire, Charlemagne created the idea of a Christian kingdom in Europe. Most of what is known about Charlemagne comes from his biographer, Einhard, a German-speaking member of Charlemagne’s court who may have been married to Charlemagne’s daughter. The concept developed in Europe from the time of Charlemagne through the Middle Ages.