Beirut — The Lebanese Army is about to be put to the test. Once split by the same factions that divide the Lebanese, it is reshaping itself to move into areas of Lebanon from which Israelis are expected to withdraw soon.
Today there is a new Army in Lebanon. And the hope of Lebanon, and to some degree the Reagan Mideast peace efforts, now rest on this Army.
There is reason for both hope and fear. The hope can be seen daily at ''the pit,'' a tent village near Beirut airport where new battalions are being trained. They shoot and march, run obstacle courses and practice hand-to-hand combat.
There is definitely a fresh team spirit among the new soldiers, now decked out in copies of United States Marine bush fatigues, and trained under the supervision of 80 Americans in the US Special Forces.
This is a long way from the days when it was once known as the hairdressers' army - when its symbols included elegant coifs, skintight fatigues, and fancy boots. One adviser admitted that the majority of officers could not pass the push-up test. And the standard joke about the presidential guards was that, after snapping to attention, they finished the salute by running fingers through their hair.
Like the country, the Army split apart during the civil war eight years ago, due to both political and religious rivalries. For the past six months, at breakneck speed, the government has promoted a program to shake up the military hierarchy and recruit or draft 12,000 new troops.
The goal is a strong Army - 65,000 by 1985 - that will be able to stand up to the many militias still marauding through the countryside.
Col. Tom Fintel, who is the chief US adviser to the Lebanese Army Modernization Program (LAMP), is enthusiastic about the swift progress. He said LAMP is two years ahead of schedule. ''The rate of volunteers really shocked us. They are coming in from all over - from behind Syrian and Israeli lines.''
And he claimed that many young members of the illegal militias have joined the Army. ''Christians and Druze [Muslims] who were fighting each other three months ago are now training together in the Army.''
But at the same time, Colonel Fintel admitted that ''there are a lot of people who are eager for the Army to fail.''
Fears about the Army's role are based on two problems:
* The military continues to be dominated by Christians, many of whom had ties with the Phalange militia accused of the massacre of Palestinians last year. Although there have been efforts to even the balance, the higher commanding positions are up to 70 percent in Christian hands.
Many Muslim groups fear deployment of the Army will only serve to consolidate the hold of Christians over Muslims.
* The reestablishment of authority on the ground has not been accompanied by a political solution that would end the cause of eight years of violence. Two Muslim leaders have recently called for a redistribution of power, now predominantly in the hands of Christians, according to population strengths of the dozen disparate sects. Otherwise, they said they would not accept the Army presence.
The new deployment will take the Army into the volatile Shouf Mountains, the heartland of Lebanon, which has been the site of sporadic clashes between Christians and Druze.
Military officials have tried to be sensitive to the fears of the two groups. A predominantly Druze brigade is scheduled to be dispatched to Druze areas, while a largely Christian brigade will be assigned to Maronite (Christian) villages.
But that, too, offers a potential flashpoint, since officials of the multinational force are concerned that young soldiers will be unable or unwilling to fight members of their own religion. There is also the more dangerous possibility that some might defect from the Army and join local militias, as happened during the civil war.
Timing is one of the question. Colonel Fintel concedes the training has progressed only to the point that the Army can defend, but not take, the offensive. The four-week basic training course also may not be sufficient to recondition the thinking of a generation of youths accustomed to thinking of members of other religions as the enemy.
Some diplomats go so far as to suggest that the various militias have actually planted forces in the new Army, in the eventuality of some confrontation.
Much will depend on Gen. Ibrahim Tannous, the new military commander appointed last December. He has taken some bold steps, particularly in removing officers considered corrupt or politically biased.
[Israeli Defense Minister Moshe Arens said that that arrangements are being made with Lebanon to provide a ''measure of tranquility'' during the transition. ''We don't consider it a mission impossible. We think that with the Lebanese Army we could work out a schedule (of withdrawal) so their capability would not be overtaxed.'']