23 But the law of nature and its Author cannot be flouted with impunity. Communism has not been able, and will not be able, to achieve its objectives even in the merely economic sphere. It is true that in Russia it has been a contributing factor in rousing men and materials from the inertia of centuries, and in obtaining by all manner of means, often without scruple, some measure of material success. Nevertheless We know from reliable and even very recent testimony that not even there, in spite of slavery imposed on millions of men, has Communism reached its promised goal. After all, even the sphere of economics needs some morality, some moral sense of responsibility, which can find no place in a system so thoroughly materialistic as Communism. Terrorism is the only possible substitute, and it is terrorism that reigns today in Russia, where former comrades in revolution are exterminating each other. Terrorism, having failed despite all to stem the tide of moral corruption, cannot even prevent the dissolution of society itself.
Source: Divini Redemptoris (Vatican.va)