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.
In the Roman Church, positive ecclesiastical laws, based upon either immutable divine and natural law, or changeable circumstantial and merely positive law, derive formal authority and promulgation from the office of pope, who as Supreme Pontiff possesses the totality of legislative, executive, and judicial power in his person. The actual subject material of the canons is not just doctrinal or moral in nature, but indeed all-encompassing of the human condition.
While the original rules included those traditional elements above, the Canon law as a whole dictates what is acceptable and what is not. (Thus the term Canon)
Common Law does not have "rules". In fact, the entire composition of Common law is based on "what has been decided before", particularly in civil suits. It's not because someone (or some God) decreed what is the ideal, but rather it's because this case is similar to the case last year. ie: stare decisis
In fact...if you go back to Henry II (birthplace of Common Law) you'll see that there was a huge fight between the law of the King (Common) vs Church Law (canon).