Shaykh Fawzaan was asked:

Question:

What is meaning of the statement of Shaykhul Islaam Ibn Taymiyyah rahimahullah “Whoever opposes the ijmaa (consensus of the scholars) then indeed he has disbelieved”?

Answer:

Yes! Allah said this:

{And whoever contradicts and opposes the Messenger (sallallaahu alaihi wa sallam) after the right path has been shown clearly to him, and follows other than the believer’s way, We shall keep him in the path he has chosen, and burn him in Hell – What an evil destination!}        Surah An-Nisa’ Verse 115

And this is an evidence upon the one who opposes the ijmaa, that indeed he has disbelieved.

Source: http://www.alfawzan.af.org.sa/node/2376

Abu Abdir Razzaaq Amjad

Note: Please refer to the following link for further explanatory articles regarding the principles of establishing Kufr(disbelief)  upon a person who may have done an act of Kufr but in one situation he HAS disbelieved and in another scenario he HAS NOT disbelieved.

The Principles of Takfir