There are billions of believers, people who think that the world has a spiritual, supernatural aspect, and some kind of all-powerful being or beings created the world and have given us instructions about how to live. That's the simple fact of theistic religious belief, a fact that cannot be hand-waved away with arguments about theological subtleties or politically correct insistence that we should respect different cultures. Billions believe that the world is more than it appears to be and that the human mind is able to reach beyond the natural world.
Modern science and philosophy should leave us in no doubt that this is a mistake. We humans have no such special supernatural abilities, and the world is a vastly larger and more complex place than was ever suggested by religious traditions. In just about every way in which theistic religions say what the world should be they get it wildly wrong.
Now, it may be that for the majority of people religious belief is relatively harmless, providing some sort of comfort and structure to their lives, but that fact should not lead us to think that religious belief is itself innocuous. Religious belief is factually a failed way to try to understand the world, yet in most of our societies religious belief is considered a virtue. That's the problem, and that's how the harm from religion arises. Religious belief is thinking without a seat-belt - the majority may drive religion safely, but all it takes is a minority to go crazy and people get harmed and even killed.
While there has to be freedom of thought, religion needs to be recognised as potentially harmful, a drug trip that can go badly wrong for some who indulge.