Letter from Islamabad

Impressions after four weeks around the Persian Gulf and in Pakistan: * Islamic fundamentalism can no longer be written off as an esoteric doctrine embraced only by crackpots such as the Ayatollah Khomeini in Iran and the Muslim Brotherhood in Egypt. It is penetrating the Western-educated elite. It extends to people who have PhDs from US universities but who nonetheless are unshakable in the error of their knowledge of the United States.

* The enormous, and still growing, Western presence in Saudi Arabia has made that traditional society even more conservative. The reaction is to draw up the wagons in a circle. Still, there are signs of change. A professor of political science asked about the consequences of the ''Ferraro debacle.'' He was not referring to the Ferraro family finances, but rather to the fact of her nomination. But a young Saudi woman (one of the few allowed to work, no doubt because of her impeccable English) said of Ms. Ferraro, ''This will be good for all of us.''

* It is difficult to see how the anachronism of Saudi society can survive intact amid all the contradictions that beset it. Modern skyscrapers go up at a frenetic pace. A Jiddah department store makes Neiman-Marcus look like Woolworth's. Saudi youth, home from school in England or the US, go out on the desert and smoke marijuana - this in a country where beer isn't available. Outside cultural influences are pressing hard. There are 70,000 Americans, plus untold numbers of other Westerners and Japanese, in addition to large numbers of Pakistanis, Indians, and non-Saudi Arabs, all in a population of 5 million Saudis. The Saudi government runs a tight ship, and Saudis are naturally conservative anyway. So far, they have protected themselves well from the winds of change, but one has to wonder how much longer they can hold out and what will follow.

* Sectarian rifts in Islam appear to be growing. There is tension in the Gulf sheikhdoms between the majority Sunnis and the minority Shi'as (Khomeini's revolution is Shi'a), the difference being a dispute over which of Muhammad's descendants is the true interpreter of the Prophet. Pakistan has lately been plagued with a succession of bloody street clashes over religion. None of this would concern the US and other outsiders except for the potentially unsettling effect in an already unstable area.

* The sectarian differences in Pakistan persist and perhaps increase, despite President Zia's movement toward what he calls the ''Islamization'' of the country. This movement is perhaps part of the same conservative forces that are so noticeable among the Arabs of the Persian Gulf. But it also seems to be a search for a national identity. Islam is the principal force that holds Pakistan together. The country, after all, was founded to provide a homeland for Indian Muslims at the time of British withdrawal from the Asian subcontinent in 1947. From the beginning it was an awkward state, consisting originally of East and West Pakistan separated by more than 1,000 miles of Indian territory. Since East Pakistan broke away and became Bangladesh, what is left of Pakistan has fewer Muslims than the number remaining in India. Nor does Pakistan, now with contiguous territory, seem to have developed a much greater sense of national cohesion. It gives the appearance, rather, of a conglomeration of disparate, often fractious, tribes and ethnic groups - Sindhis, Punjabis, Baluchis, Pathans , and others. Centrifugal forces may not have run their course.

* Notwithstanding its many problems, Pakistan seems to have done a creditable job of caring for the avalanche of refugees from Afghanistan. There has, of course, been help from the United Nations and other countries, with the US foremost, but the brunt has fallen on the Pakistanis. Nobody knows how many refugees there are. The Pakistan government says 3 million; the United Nations, 2.2 million. But in any event, there are a great many. Some are in Pakistan only for rest and recreation before returning to the guerrilla war in Afghanistan; some are being resettled in third countries, including about 2,000 a year in the US; some, so Pakistanis are beginning to fear, are there to stay. This is going to pose a problem for the world community. Assuming the Soviets stay in Afghanistan (which they are capable of doing, given the political will), at some point the traumatic fact is going to have to be faced that the refugees can never return home and therefore have to be dealt with on a permanent, rather than a temporary, basis. It is in the interests of the West to put off doing this so as to cause more trouble for the Soviets; it is in the interests of the Pakistanis to get it settled and to move the refugees somewhere else.

* Muslim, and especially Arab, frustration over US support of Israel is higher than ever, fanned by intemperate presidential campaign rhetoric and a seemingly bottomless aid program. US policy is perceived as saying one thing and doing another - disapproving but tolerating Israeli settlements on the West Bank , disapproving but tolerating the Israeli invasion of Lebanon.

A boiling over of the multiple tensions - frustration over Israel, sectarian Muslim strife, the impact of Western culture and technology, among others - seems a greater threat than the Soviet expansionism that so preoccupies Washington policymakers.