بسم الله الرحمن الرحيم
Shaykh Saalih Al Fawzaan was asked:
Has everyone disbelieved who curses the religion or rejects the sunnah?
He answered hafidahullah:
There is no doubt whatsoever in this; whoever curses the religion or rejects the Sunnah then verily he has disbelieved, because he is a denier and disbeliever in the Sunnah; also he who curses Allah’s religion, which Allah has chosen for his slaves; then he has cursed that which Allah has chosen for his slaves and legislated for them, so this one has cursed Allah -azza wa jal- there is no doubt at all of his disbelief , he who belittles Allah (and attributes deficiencies to him; Glorified and far removed is Allah from all imperfections) .
Please note that declaring a specific individual to have disbelieved and left the fold of Islaam is an affair that is to be returned back to the Scholars and it is not for other than them to declare a specific individual to have disbelieved.
For further information please see the following link: Principles of Takfir