21 Together with the victory of the revolution, though, Marx's fundamental error also became evident. He showed precisely how to overthrow the existing order, but he did not say how matters should proceed thereafter. He simply presumed that with the expropriation of the ruling class, with the fall of political power and the socialization of means of production, the new Jerusalem would be realized. Then, indeed, all contradictions would be resolved, man and the world would finally sort themselves out. Then everything would be able to proceed by itself along the right path, because everything would belong to everyone and all would desire the best for one another. Thus, having accomplished the revolution, Lenin must have realized that the writings of the master gave no indication as to how to proceed. True, Marx had spoken of the interim phase of the dictatorship of the proletariat as a necessity which in time would automatically become redundant. This “intermediate phase” we know all too well, and we also know how it then developed, not ushering in a perfect world, but leaving behind a trail of appalling destruction. Marx not only omitted to work out how this new world would be organized—which should, of course, have been unnecessary. His silence on this matter follows logically from his chosen approach. His error lay deeper. He forgot that man always remains man. He forgot man and he forgot man's freedom. He forgot that freedom always remains also freedom for evil. He thought that once the economy had been put right, everything would automatically be put right. His real error is materialism: man, in fact, is not merely the product of economic conditions, and it is not possible to redeem him purely from the outside by creating a favourable economic environment.
Source: Spe Salvi (Vatican.va)