As anticipated, questions are flying fast and thick over the proposal for the United States to join Russia in mutually renouncing the first use of nuclear weapons in the event of a new European war.

A lively debate is just what the authors, four of America's most prominent and respected authorities on national security, hoped to provoke, for they are experienced enough to know that the long-established NATO doctrine of "flexible" nuclear response-- even though it may be obsolete--is not going to be abandoned without a long and acrimonious controversy.

Reserving the right to resort to nuclear weapons may once, under different conditions, have been an effective deterrent against Soviet aggression, but the proponents of "no-first-use," now say, "Deterrence cannot be safely based forever on a doctrine that more and more looks to the people of the alliance like either a bluff or a suicide pact."

To Americans, perhaps, the most crucial point raised so far against the proposal is that the burden remains on the authors "to show how a second- use-only doctrine would leave the United States more secure."

It's a good question, and there is a good answer, which is: if nuclear war is renounced through mutual pledges of "no first use," the United States clearly has much to gain. It's not hard to see why.

Before the advent of nuclear weapons, the United States was the most secure nation in the world. It had nothing to fear from conventional war. The American continent, protected by two oceans, on the east and west coasts, and by friendly, peace-minded neighbors to the north and south of us, was essentially invulnerable to invasion and occupation.

Today, it is just as impregnable as ever, except for the threat of long- range nuclear missiles. When, and if, these doomsday weapons are renounced or abolished, the continental United States would again safely be "Fortress America," at least so far as immunity from foreign attack is concerned.

Where, however, would this leave our NATO allies? Secretary of State Alexander M. Haig insists it would be "tantamount to making Europe safe for conventional aggression." That's as exaggerated as President Reagan's rejection of a nuclear freeze on the ground that Russia has a "definite margin of superiority" over the United States in nuclear weapons.

At first glance, Soviet conventional forces appear substantially stronger than NATO's, but there are numerous mitigating factors. Russia, for example, feels compelled to keep 45 divisions on its long eastern front to protect itself against the menace of a hostile China, which has the largest ground army in the world.

Moscow also has to deploy another 35 divisions in central and eastern Europe to insure its hegemony over the Warsaw Pact satellites, four of which have already rebelled against Russian domination in postwar years.

Over half of the Warsaw Pact divisions on the Western Front are not Russian. Only the six East German divisions are rated as really reliable. So, in a showdown, Russia could confidently count on only about half of the Warsaw Pact forces for effective help. In fact, in a protracted conventional war, some of the satellite forces could well end up fighting against the Soviet Union, instead of for it.

Forswearing first use of nuclear weapons, Haig contends, would force NATO "to maintain conventional forces at least at the level of those of the Soviet Union and its Warsaw Pact allies." Not necessarily, for the defense has obvious advantages, especially logistically, in conventional warfare.

In any case, matching Russia's conventional power poses no insuperable problem. After all, the western alliance has greater resources than the Soviet bloc in manpower, industrial capacity, and general economic clout.

Also, according to former Defense Secretary Harold Brown, the NATO forces are now in better shape than at any time in recent years. Although the Warsaw Pact may be ahead, the correlation of forces, Brown says, is "either level or changing in our direction."

Haig warns that to achieve conventional equivalence with Russia, the United States would have to "put the economy on a war footing." Since Moscow is able to maintain its military establishment without going to a war footing, it's difficult to see why the United States, with its infinitely greater resources, can't do as well.

In the long run, the decision on renouncing the first use of nuclear weapons in the European theater will probably rest with our NATO allies. It's a good bet that ultimately they will take their chances on conventional war rather than invite total obliteration through nuclear escalation. The Germans and Japanese were both defeated in World War II, yet today are better off than ever before.