The Catholic Church has what is claimed to be the oldest continuously functioning internal legal system in Western Europe, much later than Roman law but predating the evolution of modern European civil law traditions. What began with rules ("canons") adopted by the Apostles at the Council of Jerusalem in the first century has developed into a highly complex legal system encapsulating not just norms of the New Testament, but some elements of the Hebrew (Old Testament), Roman, Visigothic, Saxon, and Celtic legal traditions.
What entity believes itself to have the longest standing internal mechanism of laws in Western Europe?
The Catholic Church