On Saturday night, David Brinkley commented on NBC about U.S. relations with the Soviet Union. He told a private conversation with "an unusually candid" Russian who told him how his countrymen feel about us.

They see us, Brinkley said, as being ahead of them technologically, especially in computers - and this makes us a formidable foe. However, they also think we're decadent and in danger of imminent collapse.They think crime, corruption, greed, dissension and class struggle will soon bring us to our knees. The Soviet Union won't have to do anything to emerge victorious because we will collapse from within.

They may be right. Perhaps we're too close to the trees to see the extent of the forest. The news from the Justice Department last week certainly must have seemed reassuring to that unusually candid Russian whom Brinkley quoted.

It was announced that we are at long last putting a proper emphasis on prosecuting official corruption. The FBI is conducting hundreds of investigations into activities of members of Congress and state legislatures, and hundreds more into the conduct of federal and state officials in scores of agencies. Prosecutions have been either completed or undertaken against legislators, governors, mayors, policemen and police officials, magistrates, sheriffs and even the president of a school board.

It is easy to see why Soviet observers have gotten the impression that our government is rotted through with corruption and in danger of collapse. An American would probably regard the situation as serious, but nowhere near the calamity the Soviets hope it will become.

Future generations will know which evaluation was the more accurate. Meanwhile, it is not at all reassuring to recall that many great civilizations of the past have followed the same general pattern: inauspicious birth, vigorous growth, a golden age of preeminence, and then inevitable decline as newer and stronger communities rose, coalesced and grew in power. One can assume that our turn will also come. It's only a matter of time.

If we take the hard line against official corruption, we can put off our demise - perhaps for a long time.

If voters can be educated to support national and state legislators who are motivated by a desire to benefit the public, not themselves, we might be able to revitalize our government and our civilization - and thereby outlast our Soviet adversaries.

For make no mistake about it, the Soviets' time will also come. It's just a question of which of us will be the first to reach the end of the familiar regression from prosperity to indolence, indifference, hedonism, and decline or collapse.

The outcome will be determined by the basic difference between the two countries. History will reveal which system produces better legislators and administrators: an all-powerful central authority or a democratic electorate - even on that is ill-informed and only mildly interested in its government.