Doubt is not a pleasant mental state, but certainty is a ridiculous one.
Man is a Religious Animal. He is the only Religious Animal. He is the only animal that has the True Religion - several of them. He is the only animal that loves his neighbor as himself and cuts his throat if his theology isn't straight. He has made a graveyard of the globe in trying his honest best to smooth his brother's path to happiness and heaven.
I would be presumptuous, indeed, to present myself against the distinguished gentlemen to whom you have listened if this were a mere measuring of abilities; but this is not a contest between persons. The humblest citizen in all the land, when clad in the armor of a righteous cause, is stronger than all the hosts of error. I come to speak to you in defence of a cause as holy as the cause of liberty
I don't think that the flesh is necessarily treacherous, evil, bad. It is cantankerous, and it is independent. The idea of independence is the key. It really is like colonialism. The colonies suddenly decide that they can and should exist with their own personality and should detach from the control of the mother country. At first the colony is perceived as being treacherous. It's a betrayal. Ultimately, it can be seen as the separation of a partner that could be very valuable as an equal rather than as something you dominate.