"Belief" can be used different ways. There's belief based on evidence, subject to change as evidence changes. For example, "I believe we have some milk, but let me check the fridge. No, the kids must have used it up. We're out."
Then there's belief held without evidence, or even in the face of contrary evidence, a deep-seated and important attitude. For example, "I believe that all people are created equal, and I can picture the day that we will all have the same rights and opportunities."
Theists are usually using the second meaning when they talk about their religious beliefs. If that's also how they're using it when they talk about atheism, that can be insulting to the kind of atheist who would be perfectly willing to change his mind if he checked the fridge and found a god in there. It implies close-mindedness in the face of evidence, which is praised among religious people because it's an example of keeping the faith despite temptation, but is not something that a lot of atheists enjoy being accused of.