Hell. The fear of Hell makes people disconnect their rational brain and do and think things that make no sense whatsoever

The story of Adam and Eve and the consequent subjugation of women.

The doctrine of Hell.
While it doesn't threaten me personally anymore, it keeps plenty of people in bondage, thus tethered to religions they might otherwise escape.
Christians and Muslims have all kinds of rationales, of course, why 'god' didn't create the 'hellish' place, but Satan and Man did, but any fifth grader can tell you why that doesn't wash (except many fifth graders have already been indoctrinated themselves, and have lost the ability to think rationally about it).

The Great Commission. "Go forth and preach My word unto all the lands." Christians have been using that one to invade, insinuate, and force themselves onto societies since the middle ages.

The acceptance of faith over proof and belief in superstition and magical thinking. All the things that have made humanity objectively more stupid.

The division it creates in Us and Them.

Power and control to bleed the masses.

My choice would be the deliberate making of a conclusion without evidence.
This became most evident when religions stopped mainly calling themselves a religion and instead used the word FAITH. Both words have been and continue in use. but I ( in the UK) can recall a change in emphasis some 10 to 20 years ago. They have of course been disregarding evidence for thousands of years - but is anything MORE damaging.

Suggestion: Ask a religious person “ Why the switch?"

Money and power. Religious leaders espousing theirs as the one true religion therefore all others must be destroyed. Raised Catholic, this was the continual mantra. Male domination, misogyny, prejudice, exclusion, torture of women and families. The hypocricy of their horrid secretive lives of debachery, greed and evil acts to subjugate their follower sheep.

Humanity is seriously damaged by adults who (influenced by religions & without integrity) can't/won't think critically, acknowledge truths, or determine ethical behaviours, for themselves. These 'believers' generally don't take responsibility for the consequences of the consequences of their actions.

Anyone living in fear, of a denied or hellish afterlife, as their only incentive to be good (as guided by ancient irrelevant religious texts and dogma), can't be trusted; goodness is not integral to/innate in their nature.

Those who live with disregard for responsibilities and consequences, due to their confidence in not-being-caught, expectation of ultimate absolution (through confession or matyrdom), and faith in a promised admission to a heavenly afterlife (described by religious texts and dogma) ... will not serve themselves or others well, in this one & only life. (There is a lot of religiosity in prisons.)

The universal tenant every religion has, theirs is the only true god(s). Kinda leads to some misunderstandings. Often deadly.

Tribalism.

Theocracy!

Used as an excuse to slaughter millions of people throughout history.

Manifest Destiny

That man is head of the family therefore the wife & children are his property and must obey his commands. So wide beating, child sexual abuse, physical abuse, mental abuse , etc. are all sanctioned by religion.

The 10 commandments, because it is taken too literally, with no regards for humanity when one of them is broken.

The beliefs of islam

The belief that personal testimony is somehow sufficient proof of a thing. This absolutely destroys lives every single day, and will continue to do so until enough people have learned how to determine the truthfulness of a thing or idea.

If you give this enough thought, I have no doubt you will see how it is at the heart of all religious belief.

The “God on my side” trope.

Sharia law is pretty dodgy

Afterlife. You can't posit the existence of a parallel, superior dimension without automatically devaluing this one. It's why evangelicals, for example, are willing to take a flamethrower to the Middle East to facilitate the second coming. This world doesn't matter to them.

The rape and pillaging of numerous women, children, and civilizations in the name of God.

The belief that..... God exists

The notion that religion X is "the one true faith" and must be shared with others, that others must be converted, or punished for their lack of belief. This arrogance is what allows members of religious groups to persecute people who believe in other religions, or no religion whatsoever. They are never willing to examine the consequences of their belief system because it is "THE ONE" and there is no reason to question it.