Judging by other than the law of Allah does not make one an unbeliever unless he makes that permissible or believes it to be equal or better than the judgement of Allah.
“When a person declares halâl that which is agreed upon to be harâm, or declares harâm that which is agreed upon to be halâl or exchanges the Shari’ah which is agreed upon, then such a person is an unbeliever, an apostate by the agreement of the Scholars.
According to one of two opinions, it was due to the likes of this that Allâh revealed: “Whosoever does not judge by what Allâh has revealed, such are the unbelievers.” Meaning: He holds it permissible to judge by other than what Allâh has revealed.” (Majmû’ Al-Fatâwa 3/268)
It was said to Ibn Bâz: “But whether someone has made something permissible is in the heart, and we do not know about that?!” He replied: “That is right.” (Al-Dam’atul-Bâziyah)
Ibn Taymiyyah makes clear that a Muslim who judges by other than the Sharî’ah is not considered an unbeliever unless he holds that to be permissible – just like the other sins.