For a person whose kufr is firmly established, it is obligatory to have the belief of his kufr (i.e. that he is upon kufr), and to pass the ruling upon him according to this and for the Muslim ruler to establish the punishment of apostasy upon him - providing that this person has not repented.
And whoever does not declare the person whose disbelief is firmly established to be a disbeliever – then he himself is a disbeliever.
(The exception is) if he has some doubt about that (person’s disbelief) – and then it is obligatory to try to remove that.
And with Allaah is the tawfeeq. And may peace and blessings be upon our Prophet, Muhammad and upon his family and Companions.