There are other elements that need to be considered when defining something as a religion or not in addition to the belief of a God. Any belief system that is labeled a religion usually has a belief about the afterlife and they provide a way to attaining the best possible afterlife, they usually have a moral code, they have rituals, they provide a sense of community, they have a central myth (or story), they tend to recognize something as being sacred, they tend to recognize a universal problem for mankind, and they provide a way to overcome that problem.
Atheism falls flat of possessing many of these traits… do atheists have rituals, yes, but are they accepted amongst a community of atheists who practice them together? Do atheists recognize any afterlife? Nope. Does Buddhism? Yes. Does Taoism? Not so much, but some branches do recognize that people can become immortal or gain supernatural powers, and some Taoist even believe in spirit beings and gods. Jains believe in reincarnation! I could go on….
I could go on and on.
Watch this video to learn more about what makes a religion a religion.