It's not a religion.
People consider it a religion; depending on one’s definition. If people as above define religion as “any belief people hold” such as Satanism, leftist doctrine, etc - then Islam would be a “religion”.
If people mean “a belief in God with systematic teachings and practices” then....they claim to believe in God, but their understanding, their book, their traditions and their ongoing history of butchery show that they know nothing about God, and use Islam as a political tool of tyranny and slaughter and conquest. So in that way of looking, it is not a religion.