Lebanon's unprecedentedly youthful, activist President-elect, Bashir Gemayel, is likely to be a make-or-break chief executive for his ravaged land.
Either: (1) His presidency will see the preservation of Lebanon's national integrity within its present frontiers and some restoration of a sense of Lebanese national unity. This is his professed aim. The rest of the Arab world, with the possible exception of Syria, shares it. And the United States is committed to it.
Or (2) Mr. Gemayel's presidency will see the partition of Lebanon in one way or another between Israel and Syria. That would leave Mr. Gemayel president of a rump Christian Lebanon in which he could hardly escape being (or being seen as) an Israeli puppet. The mainly Muslim remainder of Lebanon would then probably pass under Syrian control.
Under certain circumstances Israel might promote this, if it seemed to Israeli leaders that it enhanced security by giving them a client buffer state on their northern border between Israel and Syria.
Israel and Syria are foes, but history shows that foes can cooperate in partitioning weaker neighbors geographically sandwiched between them. The Germans and Russians partitioned Poland in the 18th century and again in the 20 th.
Television viewers around the world have seen in recent months not only the remarkable resilience of the Lebanese but - more important - how their sense of nationalism has grown under trial. They want all non-Lebanese out of their country - Palestinians, Syrians, and Israelis.
Thus, there is a current of national reunification to be cultivated that transcends the country's confessional jigsaw puzzle of Christian and Muslim communities. The question is: Is the youthful Mr. Gemayel, till now seen as his clan's armed militia leader rather than as a politician, the best man to seize this current and direct it so as to strengthen Lebanon and its shattered institutions?
His foes say ''no.'' Some of the more ardent of them responded to his election by bombing the homes of Lebanese parliamentarians who had turned up Aug. 23 to provide the quorum that was necessary for the presidential election to take place.
To his critics, Mr. Gemayel has two main strikes against him.
First, he has a record of initially secret but more recently open cooperation with the Israelis. Nonetheless, he disappointed the Israelis once they were at Beirut's gates by calculatedly refusing to send his Phalangists in to ''finish off'' the Palestinans trapped in the western part of the city. Israel has been the main source of arms for his Phalangist militia.
Second, back in the mid-1970s, his Phalangists were the effective spearhead in armed conflict in which the Phalangists fought as ruthlessly against Lebanese Muslim leftists as against the Palestinians. But critics sometimes forget that the combined forces of the Palestinians and the leftists were prevented from defeating the Phalangists by the intervention of the Syrians - who pose as last-ditch supporters of the Palestinians.
The Syrians then went on to secure the election as president of Lebanon under the protection of their bayonets of Maronite Christian Elias Sarkis, whom Mr. Gemayel will succeed next month.
Mr. Gemayel's foes now say he was elected under the protection of Israeli bayonets. His retort to this charge has been that there are the shadows of three foreign occupiers to rid Lebanon of: Palestinian and Syrian as well as Israeli. And the best way to get rid of all three quickly was to get Lebanon's institutions functioning again by electing a new president on schedule.
Nevertheless, the Israelis are bound to be something of an albatross around Mr. Gemayel's neck from which he sees the domestic political need to free himself. In recent weeks, he has done his best to woo leftist and Muslim foes at home - as well as Arab governments outside Lebanon. He has not had much obvious success, largely because during the Israeli siege of west Beirut, his Phalangists were still perceived as supportive of the Israelis. This, even though Mr. Gemayel kept them from doing Israel's ''dirty work'' by administering a final blow against the trapped Palestinians.
In particular, the best organized leftist opposition group, the National Movement, led by Druze chieftain Walid Jumblatt, has been unresponsive to Mr. Gemayel.
Against this background, Mr. Gemayel can hardly have rejoiced at Israeli Prime Minister Menachem Begin's congratulatory message on his election to the presidency. Mr. Begin addressed Mr. Gemayel as ''dear friend'' and said he spoke ''from the heart.''
On the other hand, there are widely believed to be Lebanese Muslims - mostly silent so far - who see in Mr. Gemayel the best man to persuade the Israelis to leave Lebanon and temper any enthusiasm in the Israeli government for the partition of their country. Their argument is that he is the Lebanese politician the Israelis trust the most because he is the one they know best.
Mr. Gemayel is in the tradition of those tough traders of the Levant coast, the Phoenicians, whose commercial expertise as wheelers and dealers goes back to Old Testament times. It remains to be seen whether these other people of the Old Testament, the Jews of Israel, have met their match in him.