THE GUNFIRE in Mecca's great mosque gives substance to Saudi Arabia's ultimate nightmare. Until last week, there was only a hypothetical possibility that religious and political radicalism might spill over from neighboring countries into that most conservative of monarchies. The possiblity is, evidently, no longer hypothetical.

Little is known, so far, about the outburst in Mecca and who instigated it. But there's a gloomy irony in the rumors ciruclating in other Moslem countries to the effect that the United States was behind it. In fact, the United States has the most explicit interest in Saudi stability and, specifically, in the steady flow of Saudi oil.

The three leading producers of the world's oil are, in order, the Soviet Union, Saudi Arabia and the United States. The United States uses all of its own oil at home, and much more. The Soviets export some of their oil, but consume most of it. Saudi Arabia is by far the greatest source of oil to those powerful economies of Western Europe and Japan that have none of their own -- and those countries are crucial to the United States both as military allies and as commercial customers. For those countries, the Saudis overshadow all other suppliers. Saudi Arabia now produces three times as much oil as Iran, four times as much as Nigeria or Venezuela, six times as much as Mexico or Britain's North Sea.

Any political disruptions suffiently severe to shake Saudi Arabia will certainly affect the smaller states in its shadow -- Kuwait, Qatar and the United Arab Emirates. Iraq has a lot of oil, but its intentions are a question mark and it may shortly be drawn into the chaos in Iran. Those countries touching the Persian Gulf now account for one-third of the world's total oil production. On the other en of the equation, two-thirds of Japan's oil comes from the Persian Gulf. So does half of Germany's oil, and two-thirds of France's. Any serious reduction in the flow from the Persian Gulf would mean severe inconvenience in the United States, but it would threaten to throw most of the other industrial democracies into a state of economic paralysis.

How did all of these countries allow themselves to slide into this desperate vulnerability to the politics of one of the world's most notoriously instable regions? For most of the years since World War II, oil was cheap and endlessly plentiful. The main worry was the constant threat of overproduction. In those days, the exporting countries only complained that their customers weren't taking enough. It seems a century ago, and yet that period ended only around 1970. By that time the industrial countries were committed to oil and had lost the capacity to move away from it easily, or quickly, or cheaply.

There's only one thing that the rest of the world can do now to reduce the danger of wider political upheaval in the Persian Gulf region. That is to cut down its imports of oil immediately. The present frantic bidding is an incitement of every political movement in the region to try to seize the oil weapon. If demand were to go down a bit and the market were to get soft, the oil weapon would not promise quite so much power.

President Carter was moving in the right direction last summer when he called for a sharp reduction in U.S. oil imports over the next decade. Unfortunately, events in the Persian Gulf are now moving faster then the president's plan anticipated.