Is it permissible for someone to say to his brothers who go forth in the cause of Allah: "You are in the Fire because you are innovators," and what is his punishment if he persists in that despite the majority of scholars saying it is not an innovation?

It is not permissible to judge a person as being among the people of Hellfire unless it is known that he died upon disbelief. Going out to invite people to Allah is an act of obedience, as long as it is not accompanied by any Shariah violation. One should not speak freely about the condition of individuals and groups except for those who combine deep knowledge with justice and piety.