It seems to me that you two are talking past each other. Here's what I hear:

ChristianKI: "Socialist movements and governments did successfully promote atheism and materialism in the populations of Europe. This is why Europeans do not tend to believe, as Americans do, that atheists are incapable of being moral." (This is a descriptive claim about history and public opinion.)

Lumifer: "We should not advocate socialism as a way of promoting atheism and materialism, because socialism is awful and Marxist ideas of historical progress are silly." (This is a normative claim about advocacy.)