Failing to Judge by what Allah has revealed is disbelief as Allah, the Almighty, says, "And whoever does not judge by what Allah has revealed - then it is those who are the disbelievers." (Qur'an 5: 44)
The correct opinion is that the Qur'anic verses which include the statements: "And whoever does not judge by what Allah has revealed …" were revealed concerning the Jews in the first place. This is why all Qur'an commentators agree that failing to judge in accordance with what Allah has revealed is considered an act of disobedience to Allah. However, the one who does so cannot be declared a disbeliever unless he clearly states that he disbelieves in what Allah has revealed, believes that it is not valid or believes that the man-made laws are better than Allah's legislation.
Furthermore, Kufr referred to in the abovementioned Qur'anic verses does not mean disbelief since, in Arabic, the word Kufr has many and different meanings. When they commented on this verse, some Companions of Prophet Muḥammad stated: "It is disbelief less than disbelief" which means that this does not render one a disbeliever. Others said that these verses do not address Muslims; but, instead they spoke of the preceding nations which denied their scriptures and legislated for themselves rulings other than those prescribed by Allah and moreover preferred the former over the latter.