June 5, 2018:
The results in 1965 were immediate and explosive. Although Althusser began to revise almost right away, many were lasting.
In my opinion the most important is his extensive and passionate demonstration of Marx's stature as founder of the science of history. He shows that sciences are ordered discourses with specific types of objects, and that Marx's historical materialism is one of these, an epochal development analogous to those of Thales or Galileo/Newton. He demonstrates a break between the pre-scientific Marx of the 1844 Manuscripts and the scientific Marx of Capital. The problematic of the 1844 Manuscripts is militantly Feuerbachian with an "injection" of Hegel. The mature Marx is free from Feuerbachianism and Hegelianism; at this time, Althusser argued that aside from its ambiguous first chapter, Capital is entirely Hegel-free. In this period he understands the break as an event with a sharply discontinuous ideological before and scientific after.
Certainly the most contentious result was Althusser's polemical slogan, "Marxism is not a Humanism." This is a perfectly benign idea: it simply means, "Marx isn't Feuerbach". "Humanism" means Feuerbach's anthropological ideology of The Essence of Man, Alienation, Species Being, and so on. The point would have been more clear if the slogan were, "Marxism is not an anthropology" — but I suppose that would have lacked the polemical juice. Althusser's "theoretical anti-humanism" became an immediate flashpoint for opposition, from the official Party philosophers who rightly identified Marx's humanism of 1844 as the cornerstone of their rightward turn, to outraged members of the intellectual public who misunderstood it as defense of Stalinism. Because it sounds, after all, like "Marxists don't like people."
So let's nail this down. For Althusser, theoretical humanism is Feuerbach's problematic of "human nature", the essence of Man, which despite its declaration of materialism is a spiritualist idealism centered on moral consciousness. "Man is that exceptional being whose attributes are the Universal, Reason, Consciousness (rational, moral, and religious) and Love." Because these attributes are eternal, there can be no reasonable theory of history in Feuerbach; this is why Marx in the 1844 Manuscripts was forced to "inject" the Hegelian idealist dialectic into the Feuerbachian anthropology in the ultimately failed and abandoned attempt to historicize it. Marx tried it, mucked around with it for a hundred pages, realized it was a dead-end, tossed the manuscript into a trunk and pushed the reset button. Marx's epistemological break leaves all this behind, when he began in 1845 to think instead in terms of the modes of production of material life. This is neither a rejection of human beings nor a defense of Stalinism: it's a rejection as un-Marxist of the ideological prehistory through which Marx traveled on his road to becoming himself.