Marxian Socialism must always remain a portent to the historians of opinion—how a doctrine so illogical and so dull can have exercised so powerful and enduring an influence over the minds of men, and, through them, the events of history.

Keynes is expressing astonishment at the historical potency of Marxian socialism despite what he regards as its intellectual defects. The remark contrasts logical rigor and literary appeal (“illogical and…dull”) with real-world efficacy: ideas can shape politics and events even when elites judge them weak as theory. By calling it a “portent to the historians of opinion,” he frames Marxism as a case study in how mass movements, moral narratives, and social conditions can amplify a doctrine beyond its analytic merits. The quote also reflects Keynes’s broader tendency to treat economic and political outcomes as driven not only by material forces but by belief, persuasion, and the psychology of conviction.