Hmm, man's law. The major problem with man's law presiding over religion (imo) is that taking supreme power out of a concept and belief and placing it in a man, or group of men, is more dangerous than leaving religion as a concept for positive guidelines because it is not concrete and is not as subject to literally having power over men. Men in positions of power easily become dictators (see the atrocities of mass murders committed under the laws of men during just the 20th century with Hitler, Stalin, Mao, et al), and if belief is placed in the laws only of men, many more people will suffer. Keeping a concept that there may be a "higher power" which must be answered to gives people at least some sliver of hope that evil men will eventually get what they deserve, whether or not that is a realistic belief.
It's difficult for me to explain this because it is not a concrete thought, but has some components of imagination that I am unable to type on a keyboard.