Seems straightforward: don't talk shit about all Muslims and pretend it's criticism of Islam. If you're railing against specific dogma, go wild. If you're insisting all seven hundred million believers must fanatically share that dogma, fuck off. Divisions within the faith are kind of contentious. Projecting third-hand knowledge of al-Shawarma militants onto Ali Patel from accounting is naked prejudice.

That goes double if you're just muttering about "Islamism" because Al wears a turban.

I think you cross the line ~~once~~ when it's not critical and just spiteful.

One of them is motivated by a rational dislike of religious delusion and oppressive thinking, the other one is motivated by its own brand of religious delusion and oppressive thinking.

"I know it when I see it" (formerly an actual Supreme Court doctrine) is kind of an embarrassing cop-out from thinking hard enough about something to actually define it, no judgment at you or anything, more in terms of "I cannot believe they decided First Amendment cases on that bullshit basis".