They claim that they alone know what is right and what is wrong and they alone also can judge that a certain act is an act of belief or disbelief.
No one is entitled to exercise a religious authority over other people. Rather, "Religion is advice". Therefore, all what we can do towards a sinner is to gently and privately extend advice to him. If he turns in repentance, then this is the desired outcome. But if he insisted to go on, then we have fulfilled our duty before Allah.
The fact of the matter is that one may incline to an act of obedience while one's heart is pure and overwhelmed with belief in Allah. This is why Prophet Muḥammad (PBUH) states, "Verily Allah does not look at your appearance or wealth but He looks at your hearts and deeds." In addition, one of the statements always repeated by the scholars of Al-Azhar reads: "Heart is the locus of Allah's investigation". So, we should do our best to purify our hearts and avoid accusing people of disbelief.
Let us all remember that what is lawful is clear and what is unlawful is clear and that declaring people disbelievers cannot and should not be made by deviant and extremist groups which have psychological and intellectual disorders.