OSTENSIBLY SYRIA is keeping the peace in Lebanon in the wake of the brutal civil war of 1975-76. Actually it is now performing something of a massacre on the Christian population in Beirut, pumping thousands of artillery rounds into the city's Christian quarter. This has something to do - no one is quite sure what - with taming the armed-to-the-teeth Christian militias that, in league with the Israelis, have been balking the imposition of control over the country by the weak Lebanese government. It also has to do with taking revenge on the particular militia whose members recently killed a Lebanese friend of the Syrian president's brother. If that sounds to you like an insubstantial set of reasons of justify the mass slaughter of civilians, we could not agree more.

It is not surprising that Israel has been in the forefront of those expressing alarm about the assault on Lebanon's Christians. It has more than humanitarianism on its mind. The Israelis, seizing the opening offered by communal strife in Lebanon, have sought for some time to maintain the Lebanese Christians as a force to neutralize the Palestinian presence on their northern frontier. That is why yesterday Israel sent jets to sound a sonic boom over Beirut and warned the Syrians that it would not stand by while the Syrian killing of Christian civilians went on.

Israel's concern is understandable. The apparent indiffence of most Moslem Arabs to the fate of the Christian Arabs is complete. Nor have the Moslems been alone in that apathy. It took six days of Syrian artillery fire in Beirut to draw a public word of concern from President Carter and the U.N. secretary general, Kurt Waldheim. The spectacle of Arabs killing Arabs, as distinguished from Israelis and Arabs killing each other, does not easily excite outsiders' concern. As evident as that is, however, Israel must know that it cannot be the guarantor of a religious community in another country. Down that path lies more conflict and unbearable political strain.

What then for Lebanon, the artificial Christian Moslem patchwork the French stuck together upon departing 35 years ago? Only American intervention kept the country together in 1958. Only Syrian intervention kept it together in 1975-76. But Syria is plainly in over its head. It lacks the political skill and perhaps even the military means to make good its longtime dream of drawing Lebanon under its rule. Its confession of failure is its artillery fire in East Beirut.

Partition, physical as well as religious, is sometimes mentioned; it might result from a process of disintergration that would develop as much by events as design. That is a decision that the Lebanese can only take for themselves - if there are any more people who consider themselves "Lebanese," rather than Christians or Moslems or rightists or leftists or whatever label they prefer. No one can be sure that there is a way to prevent continued communal warfare, which carries with it always the threat of deeper Syrian intervention, and ultimately a direct Israeli-Syrian conflict. In this instance, American diplomats are apparently acutely sensitive to that danger and are working strenously to restrain the Syrians and to hold in check any Israeli inclination to intervene.

At this point outsiders can only raise their voices against indiscriminate slaughter, no matter which group on Lebanon is the victim of the day, and lend their good offices to the reestablishment of the ceasefire that is the precondition to any attempt at restoring tranquillity to Lebanon.