President Reagan's decision to press ahead with the MX missile program while simultaneously seeking arms reduction agreements with Moscow will sorely test the theory that this nation needs to rearm in order ultimately to reduce nuclear weapons and the likelihood of war.

In a message yesterday to a Congress divided over the need for the new weapon, Reagan said that the MX "is absolutely essential to maintain America's ability to deter war." But his decision to deploy MX missiles in a "Dense Pack" formation is among the most controversial of his entire defense buildup.

Even among those who agree with Reagan that the nation needs to modernize its nuclear missile force, there is a great deal of doubt that the MX-Dense Pack plan would work as planned or whether it is any better than the Carter administration plan Reagan ridiculed, which called for shuttling 200 MXs between 4,600 shelters in Utah and Nevada.

There is also a question of whether the Dense Pack plan, because it involves construction of 100 new underground silos for the missiles, will violate previous strategic arms limitation treaty (SALT) agreements with Moscow that the administration has said it will not undercut.

The more basic disarmament issue is that the administration says the MX may make Moscow more willing eventually to agree to big arms reductions in the talks now under way in Geneva.

Yet the MX is virtually certain to be produced and deployed as a necessary modernization of U.S. forces no matter what happens in Geneva.

There are also political complications: unlike other weapons decisions in the past three decades, the MX-Dense Pack announcement comes at a time when there are spreading anti-nuclear movements in this country and in Western Europe.

The churches are heavily involved in those movements. There is intense pressure here to cut the defense budget and new leadership has taken over in Moscow.

On top of all this is the sheer technical complexity of Dense Pack, which rests on a theory that is basically impossible to test.

The theory is that by bunching 100 super-strong new missile silos close together, the first incoming Soviet missiles will blow up or knock off course those following close behind. This will theoretically allow most of the MX force to survive and strike back.

The key to the real effectiveness of MX-Dense Pack, however, will be what the Russians think of it rather than what U.S. critics think.

The administration believes that Dense Pack will make the Soviets less confident they could launch a nuclear first strike that could wipe out U.S. land-based missiles, which are the most accurate portion of the U.S. retaliatory force.

Yesterday, Reagan called the potential counters to Dense Pack "technical dreams on which no Soviet planner or politician would bet the fate of his country."

Several key senators urged that their colleagues keep an open mind about Dense Pack until all sides can testify on it in Congress.

But Reagan's decision will face formidable opposition on Capitol Hill, greater perhaps than any arms proposal he has made so far.

Reagan campaigned hard on the pledge to close a "window of vulnerability" that he believes Moscow opened in the late 1970s when it fielded a new missile force allegedly able to wipe out most of the existing 1,000 U.S. Minuteman land-based missiles.

Special commissions studying Dense Pack estimated the Soviets probably could not knock it out this decade. But eventually, Dense Pack may be vulnerable to still better Soviet missiles, requiring another escalation of the arms race and raising fundamental questions about whether any land-based missile can survive.

Some officials believe the administration made a mistake in emphasizing the window of vulnerability.

In this view, any missile attack by Moscow would be suicidal because it would kill millions of Americans and touch off a retaliatory strike with thousands of remaining U.S. submarine-based missiles and bombers.

But the administration wants MX-Dense Pack so it can stay even with the Soviets in the nuclear calculus. It wants to have in its arsenal the unspoken threat of first use of powerful, accurate missiles.

The idea is to neutralize any Soviet attempt at nuclear blackmail. But this is difficult to discuss publicly because it suggests there is a new hair trigger on nuclear war.

Among the immediate issues will be whether the new plan violates the SALT II accord with Moscow that obligates both sides "not to start construction of additional fixed ICBM launchers."

Administration officials claim that because the MX missile carries its launch equipment with it in a canister surrounding the missile, that the new underground silos are not launchers.

Defense Secretary Caspar W. Weinberger said yesterday that "it is not a new launcher because the silo cannot launch the missile." So Moscow, he said, cannot complain. But other officials say Moscow is certain to complain. Weinberger made clear, however, that "we are determined to go ahead with it."

At yesterday's briefing for reporters, deputy White House press secretary Larry Speakes was asked repeatedly whether the new missile would be a "bargaining chip" at the arms talk.

"It's certainly our hope," Speakes said, adding "but you have to deal with the realities of the situation . . . and the realities at the present moment might indicate otherwise."

Many top administration officials, however, have said privately that MX itself is not a bargaining chip. The intention is to deploy it as a follow-on to Minuteman and negotiate with Moscow about the extent of that deployment.

Indeed, in his message yesterday, Reagan held out the prospect of building even more silos if the Soviets don't agree to new controls.

"We would prefer that the Soviets dismantle SS18s," he said in a reference to Moscow's current super-missile, "rather than we build more holes."

Reagan stressed that the United States "must and will improve its forces" while remaining fully committed to the administration proposals at the Geneva talks. Those proposals contain nothing that would rule out MX as part of a reduced future U.S. missile force.

He told Congress that "many" Minuteman missiles could be removed if agreement is reached with Moscow.

In his letter and during the White House briefing, it was clear that the administration felt the Dense Pack plan was far more acceptable politically than Carter's plan had been because it is cheaper and would take up far less space in the West, Reagan's home territory.

The decision to base the missile on land will incidentally probably also help West European allies who are battling opponents in their own countries who do not want new medium-range American missiles based on European soil. Now the United States can say it is deploying missiles on its own soil as well.

In October, 1981, Reagan announced his first plan for MX. It involved basing the first 40 missiles temporarily in existing Titan and Minuteman silos.

This was rejected by Congress because the missiles would remain highly vulnerable. But at that time, Reagan defended his idea by attacking Carter's plan as vulnerable to attack and easy for the Soviets to overcome.

Yesterday, however, perhaps in an effort to win bipartisan support, Reagan was far less harsh on Carter's shell game idea of deceptive basing.

"The concept of deceptive basing, as employed in previous planning, was a fundamentally sound one for assuring the stability of land-based ICBM forces in times of crisis," the president wrote.

One other reason is that the administration might have to adopt some deceptive basing if there is no agreement with Moscow and the arms race goes on.