Atheism is a religion. It shares all the features of any other.
First and foremost, it's a system that tries to explain life, the universe and everything. That's the baseline definition of religion.
Following from that, it has sacred scriptures which may not be questioned. It has a priesthood and occasional high priests/popes. It has evangelists who seek to convert others. It has devils. It definitely has core dogmas, and punishments meted out for violating those dogmas. It offers adherents what it considers the highest and most blessed possible form of salvation...salvation from God.
Yes, it's a religion.
Atheism is not a religion and neither is theism. The words atheism/theism just describe a state of mind or belief regarding gods (whether one
has a belief or not). Atheism is a category descriptor of those people who do
not have a belief in god just like theism is just a category descriptor of those people who
do have a belief in god. The categories of atheism/theism are not religions or philosophies in and of themselves. You don't say "I believe in theism" or "I follow the theism "philosophy" (the same goes for atheism). Sure, most religions are theistic (christianity, judaism, hinduism, etc), but it is these individual religions that give the "set of explanations of how and why reality exists", not the category of "theism".
You could never say that this is what all theists believe about such and such (other than having the belief in a god), since all theistic beliefs are different based on the different religions. The same goes for atheism where you could never say that this is what all atheists believe about such and such (other than NOT having A belief in a god). Atheists may adhere to philosophies such as Humanism, or religions such as Buddhism, or something of their own makeup, but the category of "atheism" is not a religion or philosophy itself.