The very first text of Heidegger's which I happened to read after my escape to the West -- his Letter on 'Humanism' (1946) -- taught me that there is more than empty phrases to the relationship between "humanism" and Marxism. For it is no accident that Heidegger begins with Marx, though he departs from this starting point almost immediately, for a brief glance at "Christian humanism" and lands with a giant step in the age of the Roman Republic. The common ground that Heidegger finds in all these humanisms is "'the concern [Sorge]' [...] that man be drawn back into his essence [ Wesen]; for that is humanism: care and concern that man be human rather than 'inhuman,' which is to say outside his own essence. But what constitutes man's humanity?" Marxian humanism finds it in the identity between natural man and social man, Christian humanism in humanitas as opposed to deitas, Roman humanism in the contrast between homo humanus and homo barbarus.
Here Heidegger simply follows classical philologists like Werner Jaeger, when he locates a "first humanism" in ancient Rome -- thus departing from the usual textbook pattern: "humanism" in the Renaissance; "neo- humanism" in the philohellenism of the 18th and 19th centuries; and, in some cases, a "third humanism" in the 1920s and 30s. Fundamental to Roman humanism, Heidegger suggests, was the elevation and ennoblement of "virtus [...] through the 'incorporation' of paideia adopted from the Greeks" in the form of "eruditio et institutio in bonas artes"; further on Heidegger calls this process "translation." Renaissance humanism, he indicates, only repeated this process, but the "inhuman" element was now the "supposed barbarism of Gothic medieval scholasticism." He continues: "Humanism, understood historically, must always include a studium humanitatis, which specifically recalls the culture of antiquity and thus invariably turns into a revival of ancient Greece." It must be noted that Heidegger is speaking here of the "late Greek antiquity," and then only as witnessed from a Roman perspective.
As Heidegger concedes, however, "humanism" can also be understood more broadly as an "endeavor to render man free for -- and to find dignity in -- his humanity [Menschlichkeit]." In this sense, "Marxian humanism" has no more need of taking its clue from antiquity than do Christian humanism or the "humanism with which Sartre identifies existentialism".