What is the most important idea contained in the Mayflower Compact?

"Civil Body Politic" is your answer.

The Mayflower Compact was the first agreement for self-government to be created and enforced in America. On September 16, 1620 the Mayflower, a British ship, with 102 passengers, who called themselves Pilgrims, aboard sailed from Plymouth, England. They were bound for the New World.