nishtdanegesheft
The svara is pashut. Once you don’t believe a mitzvah is min hashumayim down the road it snow balls into the whole torah .This we see and saw by individuals and movements.. like reform, conservative etc. That’s the svara , I think, why the gemorrah Sanhedrin paskens ‘kol huoimar afilu ois achas eyuno mi hashomayim is kofer bechol hatorah. Because today you deny just an ois , tomorrow a whole psarshe and story ,like the OO fellas….
By the way, if they don’t believe that the torah is divine , so why bother with it a all. Just make your own set of laws or just follow the dina d’malchusay . Now you wont be bothered with all these ‘ancient back ward’ laws