But your looking at definitions based on atheism NOT being a religion. In Siquo's definition of religion the only thing missing if a ritual so a definition of ritual relating to religion can be used as it would make atheism a religion if true.
A religion is "a set of beliefs concerning the cause, nature, and purpose of the universe, esp. when considered as the creation of a superhuman agency or agencies, usually involving devotional and ritual observances, and often containing a moral code governing the conduct of human affairs."
No ritual is
required. They are just usually involved. All you need to qualify as a religion is the first part: "a set of beliefs concerning the cause, nature, and purpose of the universe."
As Atheism is not a set of beliefs concerning the cause, nature and purpose of the universe, it is not a Religion, and never will be, as by it's very nature it is the denial of Theism. Atheism can never explain the universe, all it can ever do is deny that other's people's explanations are correct.
"
Science" could possibly fall into the definition of Religion if you squint your eyes a bit, but then you run into the problem with the words "a set of
beliefs". The scientific method, by it's very nature, does
not deal in beliefs, as belief requires confidence in the truth of something not immediately suseptible to rigorous proof. The scientific method is all about subjecting things to rigorous proof.
That's not to say that belief doesn't enter the equation. The method starts with someone coming up with a hypothesis they believe may be correct, and then testing it to see if it is. But if the testing proves the hypothesis correct, it is no longer belief.
"Science" then, is not a set of beliefs concerning the cause, nature and purpose of the universe, it is a set of
proofs concerning the cause, nature of purpose of the universe. Which, fortunately (or unfortunately, depending on where you stand) disqualifies it from being a Religion.