What happens if the present opportunity for settlement in the Mideast becomes a missed opportunity? The conventional answer - the answer that passes for sophisticated wisdom in much of the State Department and the White House - is that the Arab world will be radicalized.

But, in fact, the political flow is the other way. The great unreported fact about the Moslem world is the revival of Islamic fundamentalism.

he great reported event, of course, is the oil rush that has brought billions overnight to the Gulf states and other Islamic countries such as Indonesia and Libya.

Enrichment has made the leaders of these countries - some, like the Shah of Iran, willingly; others, like the Saudi royal family, reluctantly - driving forces for material progress.

The rest of the Moslem world, if not so rich, has had to accomodate its economic style to the free-enterprise tastes of the oil sheikhs. So the dominant current through Islam is galloping modernization, and a freeing up of resources and customs.

The main resistance to this central trend comes, not surprisinglyfrom the Islamic traditionalist. They object to the easy way with money and the steady retreat from the Koran and its laws, not to mention the emancipation of women. So the whole world of Islam is now experiencing a wave of fundamentalism.

Fundamentalists provide the main opposition to Gen. Suharto in Indonesia. They tipped the scales against former Premier Ali Bhutto in Pakistan. They hold the balance between the two major parties in Turkey, and between the northerners and the southerners in the Sudan.

In Iran they have recently been particularly notable. The riots against the Shah may look to London and New York like the work of radical students. In fact, all the evidence suggests that the heart of the protests is with the mullahs, or priests. Thus the major troubles have taken place around holy shrines in Kum, Mashed, Tabriz and Yazd. The targets have been the symbols of modernization - notably banks and movie houses.

In Syria, the regime of President Hafez Assad has actually been shaken by Islamic protest. The president and his immediate entourage are members of a mystical minority sect - the Alawites. Over the past two years a dozen or so Alawites - some of them connected with the president's family - have been assassinated. The killing has apparently been done by a fundamentalist sect of Sunni Moslems based in the religious center of Hama.

A change of government in Damascus two weeks ago had as its main feature the demotion of the chief security man - former Deputy Defense Minister Naji Jamil - presumably because he was not tough enough in cracking orthodox Moslem plots against the Alawites. The security responsibility now passes to the president's brother, Riffat Assad, a figure noted for both his toughness and his corruption. It may very well be that, after eight years of stability, the Assad regime is in real trouble.

Though President Anwar Sadat clearly holds the reins in Egypt, he too faces opposition from the right. There has been a revival of the so-called Moslem Brotherhood and even more fundamentalist offshoots.

Last year Islamic extremists kidnapped and killed the minister in charge of religious properties. Last month a religious leader from Alexandria was punished for a critism - obviously aimed at Sadat - of "those who live in palaces, drive in elongated foreign cars and have wives covered with jewels."

In Saudi Arabia, leadership still rests with the seven Sudeiri brothers, heading up in the prime minister, Prince Fahd. They continue to push a pro-Western line on high oil production, which inevitably entails very rapid economic and social development.

But King Khalid, a more traditional figure, has asserted himself and become a rallying points for fundamentalist princes and groups. The upshot is slower evolution away from Kronic practice as well as only lukewarm Saudi support for expanding oil production and backing Sadats's peace initiative.

The bitterness and frustration that well up in Islamic fundamentalism obviously bode no good. Bad feelings could rise to the point of cracking the dam of authority. If the fundamentalists did take over, the result would be a fanatic anti-Western attitude akin to that shown by Col. Muammar Qaddafi in Libya.

But the chances of fundamentalist takeover are remote. The fundamentalists tend to be opposed by the military leadership, which is interested in modern weapons. They are not good at making common cause with the radicals, as Qaddafi has done, or with the Russians.

So the satus quo in most of the Arabic world look relatively solid. The most likely tendency is toward a slowing down of modernization thanks to the drag of the traditionalists. Which means there is not all that much to worry about - and certainly not from the radical quarter.