That's simple. 2 possibilities: - The religion has an official leader (for example the Pope for Catholics). He (and the hierarchy below him) gets to decide who

That's simple. 2 possibilities: - The religion has an official leader (for example the Pope for Catholics). He (and the hierarchy below him) gets to decide who is a true Catholic and who is not, and what the rules are. The line is clear. - The religion has no official leader (Islam, protestants...). In this case you can't claim that terrorists are "not true Muslims". Pretty much anyone can self-identify as a Muslim. You can however fight against generalisations claiming for example that Muslims are terrorists, or in favor of terrorist action "by default" unless they shout their opposition loud enough.