Why can I not just let the religious believe whatever they want?
It's a fair question, you'd think. And it's usually followed up by "what harm does it do" or "how does it affect you?" Answering the second answers the first.
What harm does it do?
Every Sunday, millions go and listen to one person they've given authority give their opinions based on a non-scientific, disproven book (or, rather, their interpretation of it.) These opinions then circulate out into the rest of society and affect the way we treat each other, the way we run our schools and country, the way we view the rest of the world. And every sunday they go back to be *re*indoctrinated. Bronze age myths that no longer jibe with how we know the world works are taught to children before they reach school age.
(Not to mention the economic impact of all that money *not* going to take care of food, bills, etc. 10%? Wouldn't you call that a bit of a drain on the economy, both nationally and personally?)
There's a specific directive in our bill of rights that prohibits government endorsement of any religion. That, unfortunately, is being worked around by many groups - to the detriment of equality and education. Religion makes a society *less* just. Religion forces an "us vs them" mentality on its adherents.
Personally, many Christians will demonize an Atheist. Literally, ignoring (as typical) the inherent contradiction in the statement, they'll call an atheist a satan worshipper. They'll do their best to portray their holy book, and only it, as the source for morality (a sure sign that they haven't actually read the thing,) and imply - or state outright - that if you don't agree with it, you're probably out stealing, raping people, feel free to murder and the like. And instead of actually teaching the difference, religious leaders perpetuate it. Some don't know any better. Some do.
In society? Recently we've had a lot of debates over same-sex marriage. There's no logical reason to deny it - it would make all couples of consenting adults equal, despite a genetic difference. All the arguments against it come down to "God says." Quite honestly, "God" says to murder homosexuals - which is done in some religious countries officially, and here, well, at least we label it a hate crime. Mixed-race marriage, slavery and other bias also tends to have a religious component when laws that restrict general freedom (forbidding mixed race marriage, propagating slavery) are passed.
And let's not get into the religious smokescreens in the classroom. They don't like evolution, despite the fact that it's been tested and proven repeatedly, so they try to get creationism forced in, even relabelled as "intelligent design," yelling "Teach the debate." There is no debate. Yet this wastes classroom time and forces incorrect information on students, hampering their intellectual growth in favor of a fact-free, religious viewpoint.
Imagine if this were done with astronomy. Teach what stars are, the distances involved, how they actually work - then "teach the debate" that they can fall to earth, or that they are *beings* that can come to earth and fight alongside people in battles. (Yes, those are in the bible.) Would you want that?
Religion is NOT a benefit to society, and putting mind-space aside to believe these fairy tales, to vote and support politicians who base their decisions on these fairy tales and affect local, state and national laws, hurts *everyone.*
That is why I can't just "let you believe what you want."