They perceive religion, especially Catholicism, as a limiting institution that impedes the minds of man, diverting and clouding his intellect, so that he becomes unable to see the bigger picture.W
And the 'bigger picture' is the universe and its godliness as source and energy and creator.
To them the idea of God belittles the universe--takes away its godliness--and so cramps the potential of the human being (to evolve into gods).
Atheists embrace the communist ideal and points to science as the proof of their faith, which proof is seen in the theory of Darwinian evolution, for example, (yes, they still hold to this!) and the many other proofs that science WILL find in due course.
Just like atheism, socialism is a blindness that blinkers the intellect and works in a world of immorality--if there is no God, then morality is an invention of man.
This is the very reason that socialism believes that the only moral thing is what the majority of the people want, but even in this, they deceive themselves, for they are hell-bent on indoctrinating the people to follow their evil inclinations. By this method, the socialists predetermine what the majority of the people what , whether in fact the people actually want it or not.
One will always find that the socialists are they who are readily prepared to sanction and to legislate e.g. abortion and euthanasia.
It seems to me that it is generally the case that socialism and atheism go hand in hand.