The decision here last week of 38 Moslem leaders to seek a political settlement of the crisis in Afghanistan is a turning point in their attitude toward the Soviet-installed Marxist government there.

The decision is couched in terms of a call for cooperation with the U.N. secretary general, who is to initiate contacts with the Kabul government. It represents a sharp break with the Islamic Conference's past adamant stance against the government of Babrak Karmal.

Although the Islamic Conference summit declared its "full solidarity" with the jihad , or holy war, of the Afghan rebels and warmly applauded one of its leaders here, there is a growing sense among Moslem leaders that the faction-ridden opposition inside Afghanistan has failed to develop into an effective force likely to bring down the Babrak government.

Ironically, one reason for the Afghan rebels' failure may have been the absence of significant aid from the Islamic organization. Bitterness about this was reflected by a guerrilla representative here, Sheik Mojididi, who said, "We want something in practical action, not just resolutions . . . We have only gotten 3 or 4 percent of our needs until now [in terms of] finances and weapons."

Only two Moslem countries had really provided any assistance, he said, naming saudi Arabia and Egypt. The latter, he added, "is the only country very openly and honestly giving us help."

Probably the most important ally of the Afghan rebels, neighboring Pakistan, has signaled its desire to seek a political solution while the Islamic Conference keeps up "moral political pressure" on the Soviet Union and the Kabul government as part of its strategy.

"Our approach to the secretary general of the United Nations," Pakistani President Mohammed Zia ul-Haq told the summit, "is a reflection of our determination to explore all available avenues for a political settlement of the Afghanistan crisis in conformity with the principles already laid down."

These include, according to the resolution adopted here, the immediate and total withdrawal of Soviet forces from Afghanistan, respect for the rights of the Afghan people to choose their own government and guarantees for the independence and nonalignment of Afghanistan.

The conference has set up a committee, including Iran and Pakistan, to carry on talks with the Kabul government about a political solution, a step the Afghan rebels and their supporters fear will lead to eventual recognition of it by the Islamic nations, possibly as the quid pro quo for the withdrawal of Soviet troops.

The Afghan issue illustrates problems facing the Islamic Conference Organization as an emerging political force, tilted toward the West despite efforts to strike a nonaligned posture.

Observers here were struck by the ability of Saudi Arabia, with its prestige as the cradle of Islam, vast oil wealth, and chairmanship of the conference, to weld the 37 states and Palestine Liberation Organization into a bloc seemingly with a sense of common purpose.

To be sure, Iran and Libya boycotted the summit, thus eliminating two potentially divisive forces.

But Saudi Arabia also managed to keep Syria and Iraq, whose leaders are bitter enemies, from acrimony at the summit. Jordan and Syria likewise kept their dispute outside the hall.

The Saudi foreign minister, Prince Saud, made clear that one aim of the new activist Saudi policy is to employ the force of Islamic solidarity to overcome the multiple divisions of the Arab world.

"The Islamic framework," he commented at one of his many press conferences during the summit, "will be the proper framework to improve relations among Arab nations."

Whether or not Saudi Arabia achieves this, the 12-year-old Islamic Conference implicitly is vying with the larger, left-leaning organization of nonaligned nations.

Afghanistan has become one of its prime testing grounds. A second test, familiar in the Arab world, is Israel. The Moslem countries outlined a grand strategy for stepping up pressure on Israel and its supporters by using "all their military, political, economic and natural resources, including oil."

The seriousness of their intentions has already been cast in doubt, however, by the Saudi refinement of the term jihad , which normally implies launching an armed struggle, to mean a general long-term mobilization of resources. The Saudi foreign minister explicitly rejected the use of Saudi oil as a weapon against Israel's Western backers.

This leaves diplomatic means as the main immediate "weapon." The Islamic Conference declared that its next tactical step will be to challenge Israel's credentials at the next U.N. General Assembly session and ask that it be suspended.

Whether it will succeed remains to be seen. But if the Moslem nations mobilize the more than 90 members of the nonaligned bloc on behalf of their cause, then Israel may well face a real challenge at the United Nations later this year.