Let’s make a very firm distinction between these two ideas, shall we? A belief is simply an affirmation that you think one proposition is more valid, or more accurate than others, or just a good proposition to you. Beliefs are substantiated by varying levels of support, and the level for which a person will believe something is largely personal. A belief system is a foundational set of beliefs that dictate/lead the thoughts/actions and future thoughts/actions of an individual. Beliefs can be far and in between. Random ideas that can stretch from the highly supported, ie evolution, gravity, germs, to the very unsupported, ie fairies, ghosts, gods etc.
Theism is a belief, atheism is a belief (or lack thereof). Neither one in of itself is a belief system. Alternatively, Christianity is a belief system, as Marxism is a belief system. These groups of beliefs actually have direction and depth. There is a distinct difference, and its easy to see, if your goal is not ignoring it.
So why the confusion. Well to conflate atheism with just another belief system many can toss it on the same level of control over ones lives as Christianity. As in people can claim that atheism causes action, or that people must believe other things because they are an atheist. Claiming it is belief system allows people to manufacture further complaints about atheists. And if evolution is just a belief then it can be tossed along with any other unsubstantiated belief, life one in god. In this case manufacturing a false even footing of ideas. It’s all just another silly strategy to raise up unsupported claims, not by providing evidence, but by misdefining/misleading words so that everything seems equal. It’s as pointless as it is frustrating.
Therefore to all my theist friends, don’t bother using this trick. All it does is breed circular arguments, and huge tangents, while shining a spotlight on your unsupported ideas whatever they are. If you have evidence for your god present it, anything else is just fluff.