ON Oct. 10, Fidel Castro will face an interesting test of his political strength. The Cuban Communist Party Congress meets to discuss what has been billed as the "future of Cuban communism."The debate - if it takes place - could hardly come at a worse time for Castro. He is reeling from one political blow after another, the latest being the Soviet Union's decision to withdraw its troops and advisers from Cuba and reappraise its relations with this lingering bastion of communist orthodoxy. Moreover, 600 of the 1,800 delegates to the Congress will, for the first time, be "elected by the people." That means elected Cuban-style, having undergone careful screening. Nobody believes that this carefully-constructed body is about to vote Castro out of office. But there is always the possibility that some renegade will mouth criticism of this increasingly irrelevant leader on the international scene who has brought economic misery to the Cuban people. So nervous is Castro about that prospect that he has just decreed there will be no foreign delegations to the Congress and no foreign press. That means that he can orchestrate coverage of the meeting, censoring television, radio, and the press and eliminating any mention of negative developments. Still, there can be no disguising the setbacks that Castro has recently suffered. Under pressure from the United States, without whose help it cannot survive, the Soviet Union has announced it is withdrawing its troops from Cuba. The Soviets are cutting back their economic support for Cuba. Henceforth the Cubans will have to pay in dollars for their imports. There will be no more cheap oil, no more Soviet subsidies for exported Cuban sugar. With the crackdown against coup-plotters and hard-liners in Mosc ow, Castro has lost many of his Soviet supporters. He has faced the ignominy of seeing lifelong enemies in the exiled Cuban community now being received and feted in the Soviet capital. Jorge Mas Canosa has just spent 11 days in Moscow with a delegation from the Miami-based Cuban-American National Foundation. In a telephone interview upon his return, Mr. Mas told me: "They know in Moscow that communism is over and Castro is on his way out." Mas said that the Soviet withdrawal from Cuba will include some 2,000 Soviet monitoring personnel from the electronic eavesdropping base at Lourdes. The base has until now been able to listen in on domestic American telephone conversations and other communications. Also, according to Mas, the Soviets will slow down equipment necessary to build a reactor for a projected Cuban nuclear plant. "It will never be operational," he says. Says another expert on Cuba: "The collapse of communism as an ideology has exposed Castro's bad judgment. In allying himself with the Soviets and challenging the United States he backed the wrong horse. The horse he backed just fell down in the race." How are the Cuban people reacting to all this, in addition to the economic hardship Castro's mismanagement of the economy has brought them? Secret internal Cuban polls suggest there is mounting resentment. Although last month's Pan American Games fed a certain sense of Cuban pride, there was grumbling about the expense and glitz surrounding it at a time when Cubans are scrambling for food. Observers caution that Castro has a repressive security apparatus that has heightened its cruelty to dissidents in recent months. He may have an ongoing capacity to hobble opposition. One question mark is the Army. Although Castro has eliminated generals whose loyalty he considers questionable, if Cuba explodes and collapses in civil war, can Castro count on the soldiers to open fire on civilians? Says one perceptive observer: "The Soviet Army couldn't fire on the people when the chips were down. Don't forget that the Cuban Army is Sovietized. Its senior officer corps speaks Russian, was trained in the Soviet Union. They are much influenced by what they see happening in the Soviet Union."