The present moment is as critical as any in the thousands of years of history of this ancient civilization.

What it means almost literally is whether China can repair the damage of recent upheavals and move quickly into the late 20th century. The alternative is to lapse into a pesant economy, beset by enemies on several borders.

Far from being mere political propaganda, the campaign denouncing the Gang of Four is an attempt to awaken 900 million Chinese to the great harm done by Chiang Ching, widow of mao Tse-tung, and her three co-conspirators.

The Cultural Revolution, followed by Chiang Ching's fierce drive to seize power after Mao's death, set back education, science and technology by many years. Mao's widow thought of herself as a latter-day dowager empress ferreting out any remnants of the bourgeois past and leveling a whole people to a common denominator of equality.

As one evidence of her power, she shut off a whole park and its villas on the outskirts of Peking for her private reserve.

Ideology was the sole test for entrance to universities and scientific institutions. At the end of one examination a young farm worker who turned in a blank paper was made a hero. He had defied the intellectuals and the scientific authorities.

But despite the persecution and the tragedy there were survivors, and they are now working to rebuild the educational and scentific structure. One of the survivors is Chou Pe-yuan, vice president of both Peking University and the Chinese Academy of Sciences.

A distinguished physicist who had part of his education in the United States, Chou may owe his survival partly to the fact that, at the height of the stress applied by the Gang of Four, he talked solemnly about the need to follow the wisdom of the peasants.

At a scientific conference earlier this year, he sponsored an eight-year plan under which China would reach or approach the world's advanced levels in a number of fields, increase the scientific-research personel to 800,000, build a large number of modern centers for scientific experimentation and complete a nationwide research network.

But the chief survivor is Teng Hsiao-ping. Three times he dropped into obscurity and faced the prospect of becoming a non-person, and each time he came back after Chiang Ching's attack denouncing him as a "capitalist roader."

Today he is vice premier, No. 2 after Mao's successor, Chairman Hua Kuo-feng.

Teng has set the goal of bringing China abreast of the West economically and industrially by the year 2000. It is a goal even more ambitious than that of Chou Pei-yuan. The two objectives are closely allied.

A gnarled, hard-bitten little man, Teng, at 74, is a tireless worker. He wants to draw on American technology and expertise wherever possible, and particularly in the development of China's great oil potential.

He was recognized by the late premier Chou En-lai, one of Chiang Ching's untouchable targets, as the logical leader to put China on the road to modernization.

Reports have had him at odds with Hua. Tension between them may exist, but they are performing different functions.

Hua is in the great patriotic-father tradition of Mao. They look somewhat alike, with the same beneficent good humor and cheerful visage.

Mao's image, in enlarged color photographs, is everywhere. Despite Chiang Ching and the Gang of Four, he is still the great hero and progenitor of the communist revolution. Symbolism may help to see China through this critical time.

Those who know the country well on the outside are sure of one thing: Another Cultural Revolution, with its devastating disruption, would spell the end of the China that has been able thus far to hold together a population of such amazing size.