Throughout the Communist world this week one of the truly big stories of the year, perhaps of the decade, went unreported. To the best of their ability, Moscow and its satellite capitals kept from their people the startling news that China, the most populous country in the world and once among the most fanatically communist, had repudiated the doctrines of Marx and Lenin.
The repudiation was slightly modified after the first dramatic statement. The People's Daily, the official voice of the government in Peking, had written in its edition of Dec. 7 that ''we cannot use Marxist and Leninist works to solve our present day problems.''
The next day the newspaper ran a front-page correction saying that the sentence should have read, ''One cannot expect the works of Marx and Lenin to solve all of today's problems.''
But the modification left standing the original assertion that it would be ''naive and stupid'' to cling slavishly to Marxist principles while seeking to modernize China. And it set forth a new slogan: ''Theory must be commingled with reality.''
In one sense the People's Daily editorial was only a commentary on what has actually been happening in China.
Much of Chinese agriculture was released several years ago into private hands and China's version of the enterprise system, with bountiful harvests as a reward. In October large segments of industry and trade also began to be released to the enterprise system.
It is not so very surprising to anyone who has been following the China story that the People's Daily simply enunciated what in fact has been happening. But to the people of other communist countries that enunciation is more important than the deeds that had earlier exposed China's change.
It is more important because it is taking down the bridge which has thus far connected China to the other Communist countries. It was a bridge over which the others had hoped the Chinese would some day return to the fold.
The dogmas of Marx, Lenin, and Engels are the rationale for the association of Moscow with the other communist countries.
Marxism is the ideological cement of the Soviet empire. It is the excuse under which Moscow imposes its authority on the others. It is the excuse for the ''Brezhnev doctrine'' - the doctrine that declares that Moscow has the right and the duty to see to the preservation of ''socialism'' in other countries.
As long as China continued to pay lip service to Karl Marx as the guide to the future, China could, at least in theory, find its way back into the Soviet fold.
The ideological bridge of Marxism was important to the hopes of such East European countries as Poland and Czechoslovakia, which regarded China as a friend within the fold. Peking could, as it once did for Poland, temper the wind from Moscow.
Also, the ideological bridge has figured in Western strategic thinking. It was long believed in Washington that sooner or later the common ideology of Marxism would draw Moscow and Peking back together.
It took the actual revival of the enterprise system in Chinese agriculture to persuade President Reagan that it might be worth his while to travel to Peking himself and recognize China as a potential member of the Western community.
What is left now of the ideological bridge?
Marx, Lenin, and Engels are demoted to the ranks of lesser saints, worth studying, but no longer to be regarded as infallible and no longer to be used as the sole guide to eternal truth.
There is a new teacher for the Chinese Communist Party. His name is Deng Xiaoping. His favorite injunction is:
''Emancipate our minds, proceed from reality, and seek truth from facts.''
And there is also to be a new party. Under Mr. Deng's prodding, a purge of the party is under way. The same issue of the People's Daily that demoted Marx, Lenin, and Engels reported that hundreds of party members had been charged with crimes ranging from embezzlement to bribery, smuggling, and rape. It reported that ''dozens'' had been expelled from the party.
If it were only China pulling away from loyalty to Marxism, Moscow would have less to concern itself about. But China's defection from the ideological fold is not the only example of the declining appeal of Marxist doctrine.
Communism has lost its appeal throughout much of the world. There is literally no country where one can today find spontaneous demonstrations for communism among the members of the younger generation.
Communism is still a doctrine used by some governments as an excuse for imposing arbitrary rule upon a people. ''The dictatorship of the proletariat'' is the argument used to justify arbitrary authority and personal rule. It no longer springs from the emotions of youth.
If China can turn away from doctrinal Marxism, might not others be tempted to do the same?
The People's Daily editorial of Dec. 7 made secondary news in the Western world.
The Western press had long since reported at length on the actual turn of China away from centralized Marxist-style economic planning to a more market-oriented economy.
But in the communist world this was a theological disaster. Small wonder that Moscow did its best to keep its own people from learning that China is moving away from them not only in fact, but also in theory.