Just over a year from now, when the first new Pershing II and cruise missiles are supposed to be deployed in Western Europe, the North Atlantic Treaty Organization will face its most important moment of truth in many years. The governments of the NATO nations are not prepared for that moment.

The question is whether a spreading combination of peace, anti-nuclear and disarmament movements will overwhelm the committment that NATO governments made in 1979 to deploy 572 of those missiles in five countries.

Deployment of the missiles is to begin in 1983 unless arms limitation talks with the Soviets, now underway in Geneva, produce an agreement before then. Thus far, no agreement is in sight and none is likely until these forces of public opinion are tested.

Senior officials of key NATO governments express confidence that, when the chips are down, the missiles will begin going into West Germany, Italy, England and later into Belgium and Holland. But they are also clearly worried about losing the battle for public opinion as the reality of installing these missile draws near.

How to fight that battle was among the topics of a first-of-its kind conference here recently. Under NATO auspices, a small group of NATO officials, parliamentarians and journalists from all NATO countries gathered here for informal discussions about NATO and its information policies.

In Washington, the concern was also clear. Assistant Secretary of State Richard Burt told journalists the deployment will be "a fundamental test of alliance cohesion and unity. . .one of the most important tests in alliance history." Sen. Sam Nunn (D-Ga.) called on the Reagan administration to "clear the decks of useless and avoidable disputes" with the European allies, such as the one over the Soviet gas pipeline, and concentrate on the overriding issue of NATO cohesion on the nuclear issue.

For NATO, the alliance itself may be at stake. A reneging on the deployment committment by European governments under either political pressure at home or diplomatic pressure from the Soviet Union, could unravel the 33-year-old alliance.

Indeed, the unraveling could start in the United States, where there have already been calls in one Senate subcommittee to reduce U.S. troops in Europe. European refusal to proceed with the missile deployments could create an otherwise unlikely coalition of political forces from the left and right that could cooperate to sharply reduce American contributions to West Europe's defense. It would be a coalition of liberals who opposed the missile deployments anyway and conservatives who would view any backing away from the missiles as proof that Europeans were not interested in their own defense.

For those critics of the missile deployment, especially in Europe, the stakes are also high. They see the new missiles not as a new deterrent that would make a Russian attack even less likely -- as NATO argues- -- but rather as perverse insurance that Europe will be a nuclear battlefield if war does break out.

The conference here produced a division of opinion that one would find in any bureaucracy. Perhaps more significant, it demonstrated that too many bureacrats, especially those that have security clearances, tend to get so isolated from public opinion that they are poor judges of where events are headed.

Some bureaucrats spoke of the need for reorganization, new bureaus, to make the information flow more efficient. Some officials expressed the view that the peace movements "could be handled." Others said they were being manipulated by outsiders more interested in destroying NATO than stopping the missile deployments. Others wondered what could be done about the spreading involvement of churches on the side of the disarmers. Others displayed no confidence in the media, whom they described as already committed and tilting toward the left in some important cases.

But a British diplomat, who cannot be identified under the rules of this conference, brought the real world into the austere civil service college here where the discussions were held.

While support for NATO as an alliance to defend Europe against conventional attack remains generally high in all countries, he pointed out, support for NATO's nuclear policy is confused and eroding. That is the key to the turmoil in Europe -- and it is the reason the conference was meeting here. People don't understand NATO nuclear policy, and therefore their confidence in it is eroding. Until NATO confronts these doubts and explains itself better, public opinion will remain a problem.

Toward the end, an American who works in Europe made a similar point more sharply: "We need to address what people feel threatened by, not what we feel threatened by. People feel they are more threatened by our reaction to the (Soviet) threat than to the threat itself. People feel that we are out of control."

NATO, to be sure, is not without arguments in behalf of the missile deployments, which are meant to offset more than 300 new triple-warhead Soviet SS-20 mobile missiles already targeted on Western Europe. It can be argued that the new U.S.-built missiles -- which can reach deep into the Soviet homeland -- will convince the Soviets that they could not attack Western Europe without jeopardizing their own territory, thus making war less, not more likely.

But there are also arguments and forces in Europe moving against deployment that are hard to grasp from America.

For example, as a British parliamentarian pointed out, opinion polls in Britain now show that more than 50 percent of the population of America's most steadfast ally oppose the missile deployments. A working group, led by a bishop of the Church of England has formally called for renunciation of nuclear deterrence.

Why? One reason, said a member of the European parliament in a lunchtime conversation, is that when Britain voted in 1979 vote for the missiles -- and thus to remain "coupled" to the U.S. nuclear umbrella -- it was not a vote to be "coupled" to Ronald Reagan, who was elected in 1980. Frequent talk about nuclear war during the first year of the Reagan administration created much of the problem in Europe.

A second reason was volunteered by a cab driver in London. The British already have their own nuclear weapons. But they are not enthusiastic about additional ones with a second finger on the trigger -- Ronald Reagan's.

Paradoxically, however, as diplomats here noted, Reagan's is the only voice being heard in Europe on nuclear matters. If the European populations are going to respect their governments' commitments and prevent NATO's unraveling, then European leaders, the diplomats stressed, must take the political risks and confront the groups seeking to overturn that decision.