Rather, Marxism must not only be false, but also unreasonable — not something about which anyone who has basic powers of reason could disagree. Orin Kerr seemed to suggest something like that in comments to a Concurring Opinions post:

Beling a Marxist is a lot like believing George W. Bush is the greatest President in American history. Most people will think you’re out of touch, because they think all the evidence is to the contrary. On the other hand, there are pockets of true believers who insist the doubters are the ones who just don’t get it. As a result, whether it’s discreditable depends on which crowd you have in mind.

Is this true? First, we should recognize that Marxism is a pretty big tent. While Marx himself had many beliefs which are arguably false (like the theory of history — although it’s impossible to prove claims like that false [cf. Popper], I think we have good reason to believe it’s the case), many Marxists hold modified versions of Marxism, ones that are not tied to such strong empirical claims. Indeed, I think there are many normative Marxists, who take things like Marx’s idea of alienation as normative critiques of capitalist institutions as unjust (regardless of whether or not Marx himself could take alienation for a form of injustice — a topic of some debate). On this, I strongly recommend reading Patrick O’Donnell’s comments to the above-linked Concurring Opinions post, and also Joseph Slater’s. If that’s false (that is, if there’s no alienation, or if alienation isn’t a form of injustice), it’s not obvious. (I personally happen to think it’s true, though I wouldn’t call myself a Marxist.)

Second, even those beliefs, like the theory of history and the labor theory of value, that are more likely false are probably not any more seriously false than many religions. If it’s not discreditable to believe that Christ came back from the dead, or that he’ll come back again with a bunch of angels to duke it out with the devil, it’s surely not discreditable to believe that history is driven by changes in the mode of production and that the end result is some form of communism!