Common law evolves in part based on decisions in court cases.
"Common law" in English history was the body of precedents established by courts and procedures from the Middle Ages onward. It wasn't so much that there was a codified system of laws, but there were past practices and procedures that informed legal decisions to be made in the present. The American legal system still takes this sort of approach to law, letting past precedents inform decisions on new situations that arise.
This conflict between religious and political leaders has been a constant theme throughout history and one of the most emblematic examples is the conflict between Pope Gregory VII, leader of the Roman Catholic Church and Emperor Henry IV, leader of the Holy Roman Empire. Hope this helped!
The clergy were the religious people of the Middle Ages. Following the pope, in order of rank, there were bishops, priests, monks and nuns. In the latter part of the Middle Ages, the pope, as head of the church, had much influence over the king and total control of the clergy.