Oh, fun. That old trope.
You're confusing the term agnostic with atheist. An agnostic doesn't know and doesn't care about spiritual matters. For your stamp analogy, someone who doesn't collect stamps, and doesn't care about that is an agnostic. It consumes zero time and energy.
A person who seeks to "stamp out" stamp collecting, advocates banning stamp collecting in some or all areas, and eagerly belittles stamp collectors has a hobby. That's atheism. Active hostility. (Terms like, oh, say, "brain virus" apply here.)
Atheism can also be shown to be a religion, because it passes the duck test. It holds firm beliefs on areas that differentiate religions Many of athiest's religious beliefs are held by other religions (Buddhist don't believe in a god, for example)
Deities: There are none, and man is the highest being.
Spiritual beings: There are none.
On Afterlife: There is none, this life is all there is.
Mankind: Man is inherently good (or neutral), and getting better over time.
Meaning of life: Life's only purpose is the one you give it. (see also hedonism)
Proselytizing: My belief is the right one, and it's good if other people can come to believe the same.
There's lots more, but I have better things to do today.
You can have your hobby, but just be honest about it.