What's the Catholic view on church and state?
During Christianity's earliest centuries, an era of official persecution of Christians, church and state were not only separate but opposed.
Then in the early fourth century, when Emperor Constantine became a Christian, the church and state began to visibly collaborate. The crowning of Charlemagne as emperor by Pope Leo III in 800 fostered the belief that the pope embodied ultimate authority to which secular leaders were subordinate. The ensuing Holy Roman Empire continued this perception.