Is atheism a religion? It depends on context and definitions. Most atheists bristle at the suggestion that atheism is a religious belief, and go to great lengths to distance themselves from the term.
δεισιδαιμονεστέρους is the Greek word for 'religious', as used by Paul in his Athenian speech:
Paul then stood up in the meeting of the Areopagus and said: "People of Athens! I see that in every way you are very religious. Acts 17:22
The root of this word is 'fear of the gods'.
Another greek word that is translated 'religion' is θρῆσκος, which also carries a sense of 'fear' or 'trembling' toward deities.
So an atheist, who does not believe in God, and presumably has no 'fear' of deities, would not be 'religious' under this definition and usage.
But in modern usage, and especially in legal and constitutional matters, atheism is considered a religious belief. It is protected under the first amendment, and nobody can be denied freedom of conscience, for their 'religious' beliefs. The supreme court has ruled that atheism is protected, as a religious belief, under the first amendment.
There is a phony narrative that confuses this issue: 'Christians have Religion! Atheists have Science!' This is an attempt to move the atheistic opinion/belief about the nature of the universe into a false dichotomy.. a 'religion vs science!' dilemma. But atheism is not 'science!', anymore than a theistic belief. It is an opinion about the nature of the universe. It is a philosophical belief, and is not grounded in empirical science.
I have no problem defining atheism as a 'religious' belief, by the common usage of the term. It is a philosophical opinion, and 'religious' applies. It relates to a belief about deities, and is a valid opinion.
To deny the 'religious' nature of atheism would remove it from protected status, under the first amendment. Businesses, govts, or other human institutions could deny access, if one insists on a positive religious belief in a deity.
So, why is acknowledging the religious nature of atheism a problem, for many atheists? Is it not just another opinion about the nature of man, God, and the universe?