Lebanon's warring Christian and Moslem patriarchs intensified preparations today for their first peace conference in 40 years aimed at renegotiating the nation's shredded pact for sharing power and maintaining peaceful coexistence.

Delegations that will participate in the national reconciliation conference scheduled to open on Monday or Tuesday were nearly complete with the arrival today of Sheik Pierre Gemayel, leader of the Christian Maronite Phalangist political party.

The conference's results are likely to decide not only the fate of the war-battered and badly fragmented nation, but also the degree of future U.S. support for Gemayel's son, President Amin Gemayel, U.S. and Lebanese officials agree.

The talks will open in an atmosphere of strong pessimism. Druze leader Walid Jumblatt, who arrived here Friday, told Swiss television that "everything has to be changed."

Jumblatt held consultations in nearby Montreux over the weekend with his colleagues in the opposition National Salvation Front: former prime minister Rashid Karami, a Sunni Moslem, and former president Suleiman Franjieh, a Christian opposed to Gemayel.

He said that the conference's success depended on the Gemayels and their Phalangist backers making the "necessary concessions" to establish a new democratic and nonsectarian Lebanon. Otherwise, Jumblatt said, it would mean resuming the war.

The fear, hate and mutual distrust built up among the 17 religious communities of Lebanon over the past nine years of political turmoil are so strong that the conference's participants acknowledge that uncontrollable "events" could overtake the reconciliation process long before any agreement reached here can be implemented.

In the five weeks that have passed since the Sept. 26 cease-fire agreement, "events" nearly succeeded in sabotaging the reconciliation conference.

Even before the bomb attacks on U.S. and French peace-keeping troops in Beirut last Sunday, Washington and Paris had warned President Gemayel that a failure of this conference because of intransigence by him or the Christians could seriously jeopardize their continued backing of the multinational peace-keeping force.

But the Christian Maronite minority, which stands to be the big loser in any official redistribution of Lebanese wealth and political power, is skeptical and fearful about the results of the conference.

Militia heads of the Phalangist-led Lebanese Forces, the real political and military backbone of the Maronite community, have already said they will not be bound by the results.

They argue that the "liberation" of Lebanon from all foreign forces, particularly Syrians, must precede any national dialogue, a condition that would put off for years the need to make any concessions.

This strong Phalangist opposition to any "new deal" at Christian expense may well prove the decisive obstacle to any agreement. Even among U.S. officials in Beirut, it is widely questioned whether Amin Gemayel will turn his back on his strong blood and political ties with the Phalangists.

He has publicly belittled the reconciliation conference as merely "camouflage" for what he, like the Phalangists, regards as the number one priority--the "liberation" of Lebanon from all foreign forces.

But the source of Lebanese skepticism about the conference is not limited to the expected Phalangist recalcitrance.

Even among young Phalangists who accept the need for a new deal there are serious doubts because of the history and character of the would-be peacemakers. Most are aging clan and feudal-style warlords and their sons who have built their fame, fortune and power on the shards of this nation and harbor deep-seated personal grudges against each other.

Two of the Christian warlords, Pierre Gemayel, 78, and Suleiman Franjieh, 73, have feuded bitterly ever since the Phalangists killed Franjieh's son, Tony, and his wife and child in June 1978.

Franjieh once swore he would never bury his son until his assassination was avenged. It was, but not by him, last year when president-elect Bashir Gemayel, Amin's brother, was assassinated.

Two other figures in the talks here, Amin Gemayel and the main opposition figure, Jumblatt, are personal enemies while Pierre Gemayel regards Jumblatt as a "Communist" beyond redemption.

Most of the 10 faction leaders involved in the talks have been the object of assassination attempts by one or another rival at the peace table here.

Yet these warlords, who also include the Sunni former prime minister Saeb Salam, 78, and former president Camille Chamoun, 83, are the men largely responsible for the political convulsions that have periodically seized the nation since its independence in 1943.

"Can the very people who destroyed this nation put it together again?" asked a young top-ranking Phalangist official with surprising candor. "Are these the people who are going to build the new Lebanon? We have doubts and we don't think it is going to work."

U.S. officials, in background briefings over the past month, have indicated they have their doubts, too, particularly about the Maronite leaders.

But these officials also insist that all of the feuding parties are well aware that "the alternatives are very stark."

"The alternative to reconciliation is partition," said one senior U.S. official, adding that he thought there was "a sober recognition" of this as well as the need for a new "equitable distribution" of power to save the state.

The one document on which all parties have voiced general agreement is a 10-point "Fundamentals of the Islamic Position" issued by Moslem leaders at a conference here in mid-September. It reaffirms that Lebanon is Arab and should remain a sovereign, united and independent state, maintain a democratic parliamentary republic and free enterprise system and accept "administrative decentralization" but reject any form of federation with sectarian-based "states" or "cantons."

It also calls for the elimination of "political sectarianism" in government agencies and institutions, a reference to the Christian domination of many top posts--probably the most burning issue to Moslems.

In an interview, Pierre Gemayel said he "totally agreed" with the Moslem demands, while Chamoun said that "all Christians agree to95 percent of the 10 points."

But the scope of possible agreement seems to go far beyond these general principles underlying the proposed "new Lebanon."

Various interviews with half a dozen of the invited leaders suggest they may not be too far apart on the broad outlines of a new "national pact" that might not differ greatly from the 1943 one that gave birth to Lebanon's present "confessional" system of apportioning political jobs by religion.

Both Pierre Gemayel and Sunni leader Salam, agreed, for example, that the "essential elements" of the 1943 pact should remain unchanged. But it was a deal struck between the Maronites and Sunnis largely at Druze expense.

Yet Druze leader Jumblatt, Shiite Moslem leader Nabih Berri, and Salam said they agreed a Maronite should continue to be Lebanon's president.

"We don't ask to change it from the Maronite family," Berri said earlier this month. "We accept for the president to stay Maronite to give the Christians trust."

Coming from the leader of the Shiites, who now constitute Lebanon's largest religious community and more than a third of its population, such a conciliatory attitude toward the minority Maronites may seem remarkable. Nevertheless, this is a sentiment widely shared among Druze and Sunni leaders, as well.

The issue of which Moslem sect gains which post is likely to divide the Moslems even more deeply than the struggle between Moslems and Christians unless some variation of the 1943 formula is accepted.

This provided for a Christian president, a Sunni prime minister and a Shiite head of parliament, but the Shiites and Druze are now far more powerful political forces in Lebanon than the eclipsed Sunnis.

Thus, the Moslems may have to agree to a major redistribution of posts and power among themselves that could prove even more difficult than reaching a deal with the Christians.

The Christian communities also will have problems in rebalancing their representation in the government.

There seems to be an emerging consensus on the need to create a separate senate, an idea being promoted by the Druze, who were largely left out of the distribution of top government posts in 1943 and hope to be given the leadership of this new body.

The Druze idea is that the senate should be elected on a sectarian basis, giving each religious community equal representation while elections for the parliament would be on the basis of one person, one vote, possibly with proportional representation.

The present apportionment of seats based on the 1943 agreement gives the Christians a six-to-five edge in parliament over the Moslems, although Christians now constitute about 40 percent of the population.

The Christians also hold the posts of Army commander, security and intelligence chiefs, central bank head and many other key government posts.

The Moslems want the six-to-five ratio of job distributions scrapped, but there are indications they might accept a 50-50 split of seats in parliament, although initially asking for more.

While Lebanon's non-Christians call publicly for abolition of the apportionment of positions based on religion, privately they are less strident.

Many, especially the Druze who make up 10 percent of the population, see it, as do the Christians, as a guarantee for sharing power among the communities.