If 1.8 billion, that means 77% of the world is NOT mooselimb.
In majority muslim nations there is no freedom of religion and converting from the ism can be suicidal.
I think that number its too high. W.O.G. (Wild Oriental Gentleman) arithmetic.
Muslims only remain Muslim because they are under threat of death should they attempt to leave throughout much of the world. The risk of leaving is somewhat less in the United States, because of basic non-interference with religion written into the US Federal code does give those who leave some protection, and crimes committed (murder, mutilation, crimes against property) are prosecuted (sometimes) without regard to the religious affiliation of the perpetrator.
Other countries have incorporated Sharia law into their codes, which means that crimes like apostasy and blasphemy against Islam may be and are prosecuted with the full weight of the host country. Honor killings and persecution by suppression of religious expression are overlooked when the interaction is Muslim on Muslim.