What is the ruling on calling a Muslim a disbeliever?

All perfect praise be to Allaah, The Lord of the Worlds.  I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

The Prophet, sallallaahu ‘alayhi wa sallam, told us that any one who calls a fellow Muslim a disbeliever when he is not, will have his word revert to him with the grave sin associated with it. [Muslim]

Thus, anyone who calls a Muslim a disbeliever is in great danger and must repent to Allaah The Almighty. Moreover, he should ask for forgiveness from the person whom he accused of being a disbeliever. If he is unable to ask for forgiveness or is afraid that this is likely to cause serious harm, he should ask for Allaah’s forgiveness, supplicate Allaah The Almighty for that person and praise him for his good characteristics in the same place that he accused him of being a disbeliever.

Allaah Knows best.