I see the effects of it first hand, severely crippling the potential of individuals and entire cultures. There's nothing it offers that can't be taught by basic morals which don't require you to appease an imaginary entity or repress the human spirit.
Even the leaders of religious countries know it's bullshit (see the UAE, where there are "No Displays of Affection" signs in upscale shopping malls but the streets are littered with cards advertising phone numbers for prostitution services), but maintain it as a convenient form of manipulation.
If a handful of Dungeons & Dragons players got together and started dedicating all their actions to the Drow goddess Lolth they would be called loonies, but when millions of people worship an imaginary entity for centuries and erect monuments to it and start wars in its name, apparently that's something that should be respected.
This is scary to even talk about. Not only would criticizing religion be frowned upon by first-worlders who seem to have a fetish for "exotic" cultures they have never lived in, it would put the critic at risk of physical harm in their own community.