بسم الله الرحمن الرحيم
Q: Is everyone that abandons the sunnah a disbeliever?
A: In this issue there is a detailed explanation. This is due to the fact that on rare occasions a narration may not have reached an individual, or perhaps it reached him and is sound with others but he didn’t consider it to be so. Moreover a narration reaches him which he deems to be sound, however he explains it with that which is apparent to him but not to others, or he interprets it incorrectly, and the correct interpretation is with others. In this case he is excused. Due to this shaykh-ul-Islam authored a book (1) and mention therein those excused in this regard. Consequently if he has an excuse, he is not deemed astray on the contrary it is said that he is mistaken. As for he who is deliberate then there is no doubt that he has either gone astray or disbelieved.
Translated by Najeeb Al Anjelesi
1: The book of Ibn Taymiyah mentioned here is: رفع الملام عن الأئمة الأعلام