FAA regulations part 25
The 737 MAX cannot be certified against the appropriate standard.
The relevant part (which I noticed in a post from a qualified pilot) is that when nose up is commanded at a specific amount, the Angle of Attack must increase at a constant rate.
The MAX, left to its own devices will increase the rate of increase of angle of attack due to the extra lift generated by the engine cowlings (because of their forward position on the wing and slightly nose up attitude). Therefore a system that does what MCAS does is required for certification of the aircraft. This occurs in a particular part of the flight envelope and may indeed be unstable.
Yes, the aircraft was rolled out in a hurry, and when Boeing realised they would fail a key criteria, they came up with the concept of MCAS and it was not subjected to the proper scrutiny at so many levels it beggars belief.