A significant move by President Elias Sarkis last week to reassert his authority over the Christian-dominated Lebanese army has been largely shrugged off by the country's populace, reflecting a mood of cynicism that has set in since the 1975-76 civil war.

Even before the terrorist attack in Israel Saturday, and threats by Israeli officials to launch retaliatory raids against Palestinian commando bases in Lebanon, there was deep disenchantment in all sectors of this divided country.

The disenchantment was fueled by fighting last month between the Syrian peacekeeping force and elements of the Lebanese army, aided by Christian militiamen.

Sarkis' response - his most forceful decision in 18 months in office - was to transfer, often to merely honorific posts, 60 Christian officers who had been defying his authority.

The mood of cynicism reflected public knowledge that many more Christian extremists remain in key posts in the army headquarters.This has left the central government impotent in its efforts to impose its authority on the sensitive southern border area, where Palestinian commandos have moved into new positions of strength opposite Israel.

Optimistic newspaper headlines ceaselessly predict political reconciliation between Lebanese Christian and Moslem warlords - a reconciliation that could lead to a reshuffled government to replace the present Cabinet of technocrats who lack the political muscle needed for effective administration.

The optimism often seems illusory, as when the government announced last week that repair work would finally begin on the badly damaged Beirut business center - on April Fool's Day.

As the long lines of Lebanese wait outside Western consulates to emigrate and confidence remains lacking in the business community, there is a feeling that once again Lebanon can do nothing on its own and is caught in the Middle East's propensity for violence.

The fighting against the Syrians only buoyed the more fanatical Christians who now find their Syrian former allies every bit as much an encumbrance as have Lebanon's Moslems.

People are tired, tired of viloence, tired of being terroirized and taxed by the Christian militia, tired of the Christian leadership," one Christian said, "and also scared of the breakdown in Christian leadership which is happening before their own eyes."

Last month's fighting, served to demystify the Christians, who had convinced themselves that they had won the civil war.

"Now we realize that we cannot put our lives back together the way we were," a lawyer said, " and what we see frightens us: It is the law of the jungle in some parts of the Christian areas - worse than Sabra."

Sabra is a notoriously unruly Palestinian refugee camp on the other side of the city.

Left unsaid is the realization that there is no alternative to renewing the mandate of the largely Syrian 30,000-man peacemaking force for another six months when the present mandate expires at the end of April.

Symptomatic of Lebanese impotence is the situation in the extreme south, where Israel has insisted that no Syrian troops be stationed.

Theoretically, Israel was fearful of Syrian troops so close to its northern border, but analysts long have been convinced the Jewish state prefers to maintain a level of anarchy and disorder that it can control at will.

Since Syria nad the Palestinian guerrillas have entered into another temporary alliance to counter Egypt's peace policy, southern Lebanon has once again become the sense of shifting alliances and almost daily fighting.

In recent days, elements of the pre-Syrian Saiqa guerrilla movement have moved into new positions at Ras Naqoura on the Lebanese-Israel border and in the port of Tyre. Israeli protests seemed pro forma.

No more than 500 Christian troops in Christian enclaves near the Israeli border continue to do almost daily battle with Palestinian and Lebanese leftist soldiers. The Christians are supplied by the Israelis, who also back them up militarily.

Tracked vehicles, antitank weapons and shipments of light arms have been reported arriving in the south from Cyprus, allegedly bankrolled by Libya.

The Palestinian guerrillas have again tried to police their own ranks in the south - numbering anywhere from 3,500 to 5,000 men, according to reliable sources. They did so as a result of disorders in Sidon in January which turned that normally pre-Palestinian city against the guerrilla commandos.

The Lebanese army is in no position to do much about bringing order out of chaos in the south, especially since Syria has again allied with the Palestinains.

The Beirut shooting last month is thought to have set back by at least three months plans for consolidating the Lebanese army and getting the Syrians to start their gradual withdrawal from Lebanon, a project that could take years, in any case.

By the time new recruits account for the bulk of the new army, the Middle East political equation may well have changed. In any case, as a military specialist worried out loud recently, "If that Lebanese army ever works, it will fight the Palestinians and that means problems galore."