What is the ruling on leniency in judging by other than what Allah has revealed, or deeming it lawful, given that some believe it does not affect adherence to Islam?

Whoever rules by other than what Allah has revealed commits major disbelief (kufr akbar) if he deems it permissible, or sees no harm in it, or believes it to be superior or equal to Allah's rule. As for one who rules by other than what Allah has revealed due to personal inclination or immediate gain, while knowing that he is disobeying Allah, he has committed minor disbelief (kufr asghar) and a grave sin. Allah has warned those who abandon ruling by what He has revealed, describing them as disbelievers, wrongdoers, and defiantly disobedient.