When it was first made it was considered a religion. Atheism means the lack of believing in a God. Nothing more, nothing less. If you practice Atheism you are treating it as if it is a religion there for if it looks like a duck and quacks like a duck it's a damn duck.

"first made"
There have been atheists longer than there have been theists.
You are born an atheist

You don't "practice" atheism. Theism is the belief in one or more gods. Religion literally has the same definition. Atheism is an ideology, not a religion.