Part of a meaning is not the entire meaning. Unless there's a god or supernatural beliefs involved, your definition of religion is only part the usual definitions of religion. Therefore you aren't using the word the way it is usually defined.
If you say, "For the purposes of our discussion here, 'religion' shall mean such and such, and 'atheism' shall mean this and that," then you are absolutely correct, but ONLY if you can get an agreement from forum members to use your definitions.
That isn't what we have been doing in this discussion, although you might be trying to sneak in a quasi-agreement among forum members regarding definitions that you prefer. Rather, we have, in general, been using words according to their dictionary definitions.
With regard to the dictionary definition of "religion," "atheism" fits the meaning of the word "religion." And it doesn't only fit it regarding the part that says:
something one believes in and follows devotedly; a point or matter of ethics or conscience:
to make a religion of fighting prejudice
but it also fits, to some extent, the major part that says:
a set of beliefs concerning the cause, nature, and purpose of the universe, especially 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
which is the main definition.
Notice the portion of this main definition that says, "... when considered as the creation of a superhuman agency or agencies." This portion is preceded by the word "especially," denoting that the definition,
also, stands without some kind of supreme being included.
Notice that portions following the "superhuman" part use the words "usually" and "often," denoting that the definition can stand without the portions that "usually" and "often" are referring to.
You're appealing to authority, and then you cherry-pick the meaning you want. There are many suggested definitions given by each authority, some contradictory. What makes your definition the most representative?
Nevermind, I can't change your attitude on this point and since they are all based on an appeal to authority there's no logical argument to be made.
Because of this, we can rightfully say that religion means, simply:
a set of beliefs concerning the cause, nature, and purpose of the universe.
What is interesting is that atheists are full of such beliefs in science theory, which has not been proven to be fact. Thus, atheism is definitely a religion.
Your statement is that religion is a set of beliefs concerning the cause AND nature AND purpose of the universe. Logically if any of these are not true, the statement as a whole is not true.
Atheists do not posit a "purpose" to the universe (read
https://en.wikipedia.org/wiki/Teleology).
Therefore your statement as a whole is not true, and atheism cannot be considered a religion by that definition.