The problem with religion is it primes people for believing things just based on a trusted authority saying so. There’s no evidence in support of the existence of any supernatural entities whatsoever, and there’s no evidence to support the existence of a life after death, but people believe it anyway and religion holds their “faith” to be a virtue in and of itself. You could argue that that isn’t harmful by itself, but consider that many religious people believe things that the evidence of their own eyes proves impossible, and that any idea is fair game when you treat faith as a virtue. It doesn’t matter if people today only believed the “good” parts of religion, eventually someone will corrupt their blind faith and convince them of whatever they want, like that being gay is a sin worthy of death, that trans people are evil and shouldn’t be allowed to exist, that your pastor is totally a great guy and you should donate money to the church and totally trust him alone with your kids. The dangers of religion are in teaching people to stop thinking for themselves.