Marxism is a political belief based on economic theory. Not all atheists subscribe to it.
It doesn't define atheism any more than Bugs Bunny defines rabbits. (Happy Easter):)

Remember also that Ayn Rand followers, it they are devout, purport to be atheists as well.

Jack, some scientists are claiming that "nothing" or empty space is matter that is of a quantum nature and that we can't see it. Energy may be a form of matter.

One could argue that dogmatism is of a generalized "religious" nature and therefore
Bolshevik Communism might be loosely defined as "religious" since its adherents
have a dogmatic devotion to it.

The logical fallacy prevails. You can't prove a negative. You can argue in favor of Santa Claus or the Easter Bunny but if you can't prove it, it becomes an irrational belief.

Does it serve a social purpose? Maybe sometimes it does. Some more enlightened religious groups like the Quakers or Unitarians (many who are atheists) do important social work as do other offshoots of established religions. I, for one, am sympathetic to Liberation Theology, anathema to the Pope and Catholic hierarchy.

Atheists are atheists because there are so many different kinds. They are generally opposed to dogmatism with proof wherever it rears its ugly head. BTW, there are
different "gods" that Christians and other faiths believe in as well.