The separation of church and state is a relatively new development. When Emperor Constantine gave freedom of religion to the Roman Empire in the Edict of Milan in 313 AD, the Church was able to come out of hiding and into the public. Constantine gave many government buildings over to the Church for its use. In fact, Christianity became the state religion of the Empire. It was hard to distinguish the role of government from that of the Church.
As time went on, the relationship between church and state intensified. For many years there was only one religion, the Catholic Church. Everyone in Europe was Catholic. The height of earthly power in the Church came under Innocent III. All rulers and nobleman were subjected to the pope and looked to him for guidance. In addition, the pope was also a leader of a country, the Papal States. He looked to strong Catholic rulers for help from time to time, for example, when the Papal States were being invaded. With their aid came conditions. One of the conditions was that the emperor of the Holy Roman Empire would be in attendance at papal elections. Rulers also had a say in who could be bishops of diocese in their realm. The division of church and state was so slight that the emperor had the right to approve who could enter a religious community or order.
This interference was at its peak when the king of France kidnapped the papacy and brought it to Avignon, France. For over seventy years, the center of church government was not Rome but the back pocket of the king in Avignon. He regulated Papal elections, which undermined the central authority and power of the Church.
After the American Independence and the establishment of a new Catholic diocese, the Holy See conferred with President George Washington on his choice for the new bishop. The American government, which is built on the principles of the separation of church and state, said it was none of their business. For the first time, the Church did not need government approval on its choice of bishop. Over the years this would become the norm. Only in Communist countries would government still interfere in Church policy.