In Islam, Heresy and blasphemy are very dangerous. In Pakistan 2 high level officials have been assassinated for attempting to eliminate or speaking against Pakistan’s severe blasphemy law.
In the section on Islam, Belloc argues that Islam is actually a Christian heresy, not that I necessarily agree with him.
It’s not about heresy in Islam, if that’s what you thought.
But yes, heresy and blasphemy can be very dangerous in Islam.