To propose that "Atheism is a religion" implies that it takes as much faith to "believe in" Atheism, which is a logical impossibility, as it does for any faith-based (read: accepting ideas despite a lack of evidence) religion. The OP's original definitions include the word "belief", which is a loaded word out of the gate.
Here's the difference between believer and non-believer:
The religious mind has a confirmation bias. I'd link that but I can't for another 14 posts. They will admit to it, albeit not always so easily. That confirmation bias is, "God is the answer to everything."
"What caused life on this planet?"
"God did."
"What happens when we die?"
"God judges you."
"What is love?"
"Jesus is love."
These may sound oversimplified, but for the religious mind, imagining a different answer, despite evidence to the contrary, is just short of impossible. Their mind always returns to that answer.
The mind of logic, reason, and deduction of evidence has no such bias. I let my answers take me where that leads, no matter what the result. If there was repeatable, verifiable, non-biased evidence of God's existence, I would conclude that there is a God, in a second. I don't call that faith; I don't call that "belief" because I'm always testing it. Always. It's in a constant state of flux for me. Not agnostically so, however.
The religious mind, however, is provided with evidence to the contrary (of their beliefs) on almost A DAILY BASIS, yet they still believe. That's the difference.
People of Faith reading this right now, I'm sure, are thinking "But there is evidence for God's existence."
No, there isn't, but that is not pertinent to this thread.
So, bottom line Atheism is not a religion because it does not accept things on faith. Yes, there are some people who think of Atheism as a religion, in that they are fanatics about it, but I prefer to call myself a freethinker more than I am an atheist. My freethinking just got me there.