Marxism was one of the first versions of secular humanism and remains a classic in its own right, but at this point it is likely to get kidnapped by the social darwinist ideology of the new atheists, darwinian fundamentalists and the proponents of reductionist scientism.
In reality the path to socialism don’t require any opinion or new dogmas on such subjects, although it might be nice is socialist could show some intelligence in the realm of philosophical history, and some sense of the illusions of darwinism. The socialist left however has no real business legislating atheism even if a critique of religions is or is not relevant.
But a shallow secular humanism or the new atheism are likely fatal to a socialist future which must do such questions with considerable dialectical subtety, or not at all if too busy moving along the vein of socialist transformation.
Source: Challenging religion…then secular humanist darwinism…le plus ca change//Gregory Paul on the rise of nonbelief in the U.S.: it’s happening faster than you think « Why Evolution Is True – Darwiniana