Everyone hates hearing “your religion is wrong”. Religion is extremely important to people, and being told your entire world view is incorrect can really sour your day. I think the difficult part for Atheists is that when people hear “Atheist” they automatically think “here is someone who denies everything I love and hold true.” Well, yes, technically, Atheists do not believe in all the religious stuff you do, but that doesn’t mean they’re going to attack you for it at every instance. (Yes, there are Atheists who might seem very loud and combative about it, but that’s because we live in a society that demonizes and persecutes them. They’re lashing out because of this)
To believe one things inherently means you disbelieve another. If you say you’re a Christian, then you are also saying Muslims, Jews, Atheists, Pagans, Buddhists, etc, are all wrong. If you’re a Muslim, you believe all the others are wrong. I think the reason people don’t freak out when someone tells them they are another religion, like Jewish or Muslim, is that at least you have god in common. You might disagree on the particulars, but at least you have some common religious ground. Atheists don’t have that benefit. They are the anti-thesis to all things religious.
Yeah Atheists will disagree with you on just about everything religious, but the common ground you can find with them is in caring for your neighbor. Many Atheists, just like many believers, are normal, kind people, and there is a lot we can do together if we just get past the bickering.