From the biography of the mathematician Paul Erdös, The Man Who Loved Only Numbers (p. 46):
Pythagoras saw perfection in any integer that equaled the sum of all the other integers that divided evenly into it. The first perfect number is 6. It’s evenly divisible by 2, and 3, and it’s also the sum of 1, 2, and 3. The second perfect number is 28. Its divisors are 1, 2, 4, 7, and 14, and they add up to 28. During the Middle Ages, religious scholars asserted that the perfection of 6 and 28 was part of the fabric of the universe: God created the world in six days and the Moon orbits the Earth every twenty-eight days. St. Augustine believed that the properties of the numbers themselves, not any connection to the empirical world, made them perfect: “Six is a number perfect in itself, and not because God created all things in six days; rather that the inverse is true; God created all things in six days because this number is perfect. And it would remain perfect even if the work of the six days did not exist.”
Biblio: Hoffman, Paul (1998), The Man Who Loved Only Numbers: The Story of Paul Erdös and the Search for Mathematical Truth (New York: Hyperion).