The wave of religious fundamentalism now sweeping the Islamic world describes the broad scope and enduring character of the challenge posed to this country by Ayatollah Khomeini. The plight of the hostages is the most immediate business, of course. But after that comes the matter of dealing with a regime that menaces international security. Then comes the long, drawn-out effort to rebuild respect for the United States around the world.

The hostages now enter a period of maximum danger. The frenzy for martyrdom that accompanies the first 10 days of the holy month, Muharram, reaches a climax on Friday. There follows a plebiscite in which the ayatollah needs to build maximum support for his Islamic constitution. During this time the hostages are most likely to be harmed or put on trial.

The best Washington can do, at present, is to play it cool. The more so since the condition of the shah may make it possible for him to leave the country in the next fortnight.

Unfortunately, though for understandable reasons, the adminstration opened the crisis by forswearing the use of force and moving to bargain on the hostages. In response, the ayatollah expressed some doubts as to the president's guts.

Carter felt constrained to assert that force might be used if harm came to the hostages. The administration also sought something it had previously avoided -- formal action on Iran by the United Nations Security Council. So circumstances have forced an escalation of rhetoric at just the moment when voices ought to be still.

The ayatollah, accordingly, may emerge from the present confrontation with enhanced prestige at home. But if nothing else, he has been exposed to the world as a genuine menace to peace -- not just because of the hostages, or his harsh comments about the United States. Far worse, he has thrown lighted matches on dry leaves by mobilizing Islamic fundamentalism against the regimes in all the neighboring states -- Turkey, Pakistan, Iraq and the monarchies, including Saudi Arabia, of the Persian Gulf. As never before, the source of much of the world's oil is now at hazard.

Mixed up in the delirious crowds he brings into the streets, moreover, are Marxist elements ready in the event the ayatollah falters to take power and turn it to the account of Moscow. So helping moderate Iranians rid themselves of their present government has become an American interest of high priority.

Elements hostile to the Khomeini regime undoubtedly exist all over Iran. Secular groups in Tehran have demonstrated against his theocratic tyranny. The Kurds and other minority and tribal groups around the edges of the country have shown restiveness.

But it is idle to think that the regime can be toppled without action by the Iranian military. It is equally dreamy to suppose the Iranian military will move without some support from the United States.

In the past the Carter administration had hoped to draw the Islamic regime toward sanity by working with secular elements of a social democratic cast grouped around former prime minister Mehdi Bazargan. Now that group has been shattered. It remains to be seen whether the administration has the steeliness to abandon the well-nigh eternal American hope for a broad middle-of-the-road democratic coalition in order to face the ugly realities of Iranian politics.

Americans will not be alone in watching. Iran's neighbors, ad regimes friendly to this country all over the world, will also be looking carefully.

Many of those feel unsure of themselves and doubtful about American protection. The cautious way in which the Saudi regime handled the shoot-out in the Grand Mosque in Mecca does not announce total self-confidence. The slow response of the Pakistani regime to quell the siege of the U.S. Embassy in Islamabad does not argue total faith in the United States.

The effort to rally former friends is necessarily going to be long, slow and hard. It involves the rebuilding of American forces around the world. It requires the jettisoning of a moralistic approach to world politics in favor of a strategic approach.

In making the shift, the Carter administration is going to need all the help it can get. There is no way it can fix blame upon others for what has happened. So it behooves the president to stop the petty skirmishing with supposed rivals and start leading the country as a whole.