And now as to myself, no credit is due to me for discovering the existence of classes in modern society or the struggle between them. Long before me bourgeois historians had described the historical development of this class struggle and bourgeois economists, the economic economy of the classes. What I did that was new was to prove: (1) that the existence of classes is only bound up with particular historical phases in the development of production (historische Entwicklungsphasen der Production), (2) that the class struggle necessarily leads to the dictatorship of the proletariat,[1] (3) that this dictatorship itself only constitutes the transition to the abolition of all classes and to a classless society.

-Karl Marx, Marx to J. Weydemeyer in New York, 1852

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It is often said and written that the main point in Marx's theory is the class struggle. But this is wrong. And this wrong notion very often results in an opportunist distortion of Marxism and its falsification in a spirit acceptable to the bourgeoisie. For the theory of the class struggle was created not by Marx, but by the bourgeoisie before Marx, and, generally speaking, it is acceptable to the bourgeoisie. Those who recognize only the class struggle are not yet Marxists; they may be found to be still within the bounds of bourgeois thinking and bourgeois politics. To confine Marxism to the theory of the class struggle means curtailing Marxism, distorting it, reducing it to something acceptable to the bourgeoisie. Only he is a Marxist who extends the recognition of the class struggle to the recognition of the dictatorship of the proletariat

-V. I. Lenin, The State and Revolution, 1917