Only a month after announcing that he would continue abiding by the 1979 treaty on strategic nuclear weapons (better known as SALT II), even though the Senate has never ratified it and in spite of repeated Soviet violations, Ronald Reagan finally found the courage the other day to say no.

Courage, like cowardice, being infectious, Reagan on the very same day said no to another document that would have acquiesced in Soviet violations of the 1975 Helsinki accords on human rights.

Courage? Why should it have taken courage for Ronald Reagan, of all presidents, to respond forcefully to broken Soviet promises? The reason is that doing so meant standing alone, not only against the Soviet Union but also against all our allies (not to mention a large body of opinion at home).

To be sure, Ronald Reagan has a reputation for ideological purity. He is also seen, especially by the allies, as a quintessential loner -- a ''cowboy.'' But what he has really shown himself to be is a man with a great eagerness to be liked and a politician with an almost insatiable appetite for popularity. For such a man and such a politician, the temptation to continue going along must have been enormous. So, conversely, was the courage required to overcome that temptation.

For it is no exaggeration to describe the situation as one of standing alone against the whole world.

Thus in Bern, Switzerland, representatives of 35 nations spent many weeks reviewing the record of compliance with certain of the human-rights provisions agreed upon at Helsinki in 1975. With the help of a series of brave speeches by the chief American delegate, Michael Novak, it was made clear to all concerned that the Soviet record on personal East-West contacts has grown worse in the past 10 years. To cite only one example of many, since 1975 new laws have been passed under which Soviet citizens can be punished for entering into a variety of casual contacts with foreigners.

Nevertheless, a last-minute ''compromise'' document was drawn up that would have done nothing to correct violations of old principles. Nor did it provide any means of ensuring that new promises would be kept.

Novak refused to blink at what he called ''the real problem -- the problem of compliance with existing documents.'' In addition, he judged that the document presented at Bern would weaken rather than strengthen Soviet compliance. Consequently, with the full backing of the administration, he declared that the United States would not assent. Few of the other 34 nations present at the meeting had any illusions about Soviet compliance. Yet most of them were fearful of seeming ''confrontational,'' and all gave their consent.

No such precise tally exists in connection with Reagan's ''vote'' against SALT II. But the NATO foreign ministers, at a gathering in Canada, all expressed strong disapproval of what one of them described as this ''profoundly disturbing development.''

It is not because our allies are unaware of Soviet violations of SALT II that they are so desperate to keep it alive. As with the Helsinki accords, everyone knows that, even giving the Soviets the benefit of every doubt, their record of compliance is highly questionable.

After all, the Soviets have introduced two new missiles since 1979 when only one is permitted under SALT II. They have resorted to forbidden coding devices that frustrate the verification procedures stipulated by the treaty. And they have been guilty of a score of lesser violations.

The Soviets themselves, and some of their apologists in the West, have tried to explain away these violations. But the NATO foreign ministers have not really been fooled. In fact, the Canadian representative, while deploring Reagan's decision, went on to hope that the Soviet record will improve enough to induce the president to change his mind again.

Why then, as the Psalmist asked in a not entirely dissimilar context, ''do the nations rage"? Why are they united against the U.S. announcement that from now on our deployments in the field of strategic weapons will be governed by considerations of military security and not by the need to stay within the limits set by SALT II?

The answer to this question is suggested in the same Psalm: because ''the people imagine a vain thing.''

The vain thing the people imagined then was that their leaders could defy the power of the Lord. The vain thing the people imagine today is that their leaders can defy the laws of political reality. Specifically, they imagine that the world can be made safer through paper arrangements with a totalitarian regime.

Never mind that this idea has been discredited by experience, first with Nazi Germany, and then with the Soviet Union itself.

Never mind that in the '60s the Soviets used the arms-control process as a means of catching up with us when we were ahead.

Never mind that they then used it in the '70s as a screen behind which they could pursue the military superiority they have always been determined to achieve.

Never mind that they are trying to use it now to prevent the United States from developing a defensive system that would deny them the advantages of their overwhelming offensive capability.

Reminding himself of all this, Ronald Reagan finally summoned the courage to say no to the people who insist on putting their faith in so vain a thing as the arms-control process and their leaders who cater so shamelessly to that empty faith.

Now all he needs is the courage to hold firm as they rage so furiously together against him for daring to act on the truth.