In his 1899 book *Die Voraussetzungen des Sozialismus und die Aufgaben der Sozialdemokratie* (translated as *Evolutionary Socialism*), Bernstein argued that Marx's predictions of capitalism's inevitable collapse were being disproven by events. He pointed to the improving conditions of the working class, the diffusion of property ownership, and the success of the cooperative movement as evidence that capitalism was not failing. From this, he concluded that socialism could be achieved through gradual, parliamentary reform and democratic means rather than a single, catastrophic revolution.

Orthodox Marxists, following the revolutionary tradition of Marx and Engels, believed that capitalism would inevitably lead to a crisis that would be resolved by a proletarian revolution. Bernstein challenged this by arguing that the working class could achieve its goals through the ballot box and by building political power within the existing system. He saw the social democratic party as a vehicle for reform, not a revolutionary vanguard, making him the key figure in the 'revisionist' controversy that split the international socialist movement.

Bernstein was a fierce critic of the Bolshevik Revolution of 1917, seeing it as a violent and authoritarian perversion of socialism. He argued that the establishment of a dictatorship contradicted the democratic principles he believed were essential to socialism. He used his influence as a respected SPD politician to discourage German socialists from following the Russian example, viewing the parliamentary republic as the only legitimate path to progress.

Bernstein was a friend and correspondent of Friedrich Engels, though their relationship could be contentious. In the 1890s, Engels expressed frustration with Bernstein's initial reformist arguments. Despite this, Bernstein was a committed socialist who spent his career attempting to 'update' Marx's ideas to fit the changing conditions of advanced capitalism, a project he saw as staying true to Marx's scientific method, even as he diverged from Marx's conclusions.

After the war, Bernstein served briefly as a Secretary of State for Economy and Finance in 1919. He was a member of the Independent Socialists during the war due to his anti-war stance but returned to the SPD afterward. He strongly opposed any attempt to transform the November 1918 political revolution into a social revolution, believing that the new parliamentary republic provided the framework for the progressive, peaceful reform he had always advocated.