President Reagan has traveled a long and difficult journey to achieve the agreement in principle on a new U.S.-Soviet arms treaty that he announced in the White House briefing room with great fanfare on Friday. Along the way, he has learned to treat the Soviet Union with respect.

At his first news conference, on Jan. 29, 1981, Reagan was asked whether "detente" was possible or if he believed that "the Kremlin was bent on world domination." His answer ushered in one of the chilliest periods in U.S.-Soviet relations since the beginning of the Cold War.

"Well, so far, detente's been a one-way street that the Soviet Union has used to pursue its own aims," Reagan said. "I don't have to think of an answer as to what their intentions are; they have repeated it." He then asserted that all Soviet leaders believed "that their goal must be the promotion of world revolution and a one-world socialist or communist state, whichever word you want to use . . . . The only morality they recognize is what will further their cause, meaning they reserve unto themselves the right to commit any crime, to lie, to cheat, in order to attain that . . . . "

Even more revealing than the stark language was Reagan's admission that he didn't have to think about the answer. Guided by a reflexive anticommunism that substituted for policy, Reagan casually denounced the Soviets, as if trying to keep in practice. He called them "the focus of evil" in an interview and the "evil empire" in a speech March 8, 1983, when he defended the comments of his first news conference, adding the insult that he was only quoting something Lenin said in 1920.

Times have changed. Reagan began last week in the White House Rose Garden where he stood before U.S. and Soviet flags and witnessed the signing of an agreement designed to reduce the risk of accidental nuclear war. He concluded it with his announcement in the briefing room of an accord that will remove medium- and shorter-range nuclear missiles from the arsenals of both superpowers. Both sides hope this will pave the way for a more significant pact reducing strategic nuclear arsenals.

The odds on such a far-reaching treaty are long. Profound U.S.-Soviet differences on missile defense may leave the task of reaching a strategic arms agreement to Reagan's successor. But the process of arms control has survived.

Reagan's liberal adversaries give Soviet leader Mikhail Gorbachev credit for this progress, and he may deserve it. Thousands of Soviet citizens remain unjustly imprisoned, and Gorbachev has not ended the brutal invasion of Afghanistan. But, as Reagan acknowledged in Los Angeles last month, the new Soviet leadership has begun diplomatic initiatives and internal "movement toward more openness, possibly even progress towards respect for human rights and economic reform."

However, Reagan also deserves more credit than he is being given. He has kept his campaign promise to increase military spending. He has used the Strategic Defense Initiative, whatever else can be said about it, to prod the Soviets back to the bargaining table. And he has learned in dealing with Gorbachev and his emissaries that leaders of other superpowers, even communist ones, prefer to be treated like human beings.

The president's muting of his habitual anticommunist rhetoric is not a sign that he has gone soft on communism. It is a signal that he is now willing to deal with the Soviet Union as a legitimate, rival superpower rather than an immoral gang of liars whose only aim is conquering the world.

In a celebrated essay after World War II, writer George Orwell discussed the corruption of language and concluded that "language can also corrupt thought." What Americans say about the Soviets is important, to them and to us. The Soviets respect frank talk about human rights or Afghanistan, but they are more likely to pay attention to a president who acknowledges their genuine national interests and treats them as equals rather than one who gratuitously insults them.

Reagan has made this transition, without yielding on essential positions. A grown-up quality has returned to U.S.-Soviet relations. In the most vital arena of U.S. foreign policy, Reagan has matured.

Reaganism of the Week: Asked at the White House if he was in too much of a hurry for a summit and an arms treaty, the president said: "I don't know of anything in my life I waited over six years for."