Humanity mourns for the victims of the carnage in Sabra and Shatila. It should also pray that all the participants in the terrible drama taking place in Lebanon - Christians, Muslims, Jews - will subordinate their anger and fears to higher impulses of reason and restraint. The need of the hour is not for more sectarian hatred but for the healing balm of forbearance and compassion. If that seems a utopian dream in a land torn by savage conflict, the Lebanese people still have it in them not to let forces of destruction and hate frustrate their deep longing for a unified, free, independent country.
That is the objective to which all efforts, above all of the United States, must be vigorously bent.
The full story of Sabra and Shatila has yet to come to light. The Lebanese Christian gunmen who slaughtered hundreds of Palestinian men, women, and children will always bear the awful burden of defiling the religion by which they profess to live, a religion which calls for loving even one's enemies. They have made a mockery of their faith, and their actions deserve the world's revile.
That Israel also bears an awesome weight of responsibility cannot be denied either. Perhaps it did not want anything like this to happen. But, as President Reagan stated, Israel blocked efforts of the Lebanese Army to take control of west Beirut after the PLO evacuation. It claimed it would keep the peace. Yet Israeli forces apparently permitted Christian militiamen to move through their lines into the Palestinian camps.
Israel's defiant occupation of west Beirut is deplorable on many counts:
* It is a serious blow to the US plan for a Middle East peace. This appears to be precisely the intent of Prime Minister Begin, who peremptorily rejected Mr. Reagan's initiative from the outset and now seems determined to undermine it - at whatever cost.
* It means that for the first time in history Israel is occupying an Arab capital, a fact which belies the self-drawn image of Israel as the embattled victim of Arab encirclement. This humiliation to Arabs, along with the massacre of Palestinian refugees, will make compromise and reconciliation even more difficult.
* It deprives the Lebanese people of the right to work out their political future free of outside interference. There is little doubt Mr. Begin put troops into west Beirut in order to influence the formation of a new government after the murder of Bashir Gemayel (despite the fact that Muslims and Christians had made a genuine show of wanting to put the country together again). This may suit Mr. Begin, who wants a Christian-dominated buffer state closely linked with Israel, but it risks sparking a bloody civil war in Lebanon.
* It puts Israel and the United States on a collision course. However much the US seeks to safeguard Israel's existence and security, it now finds its policies in conflict with those of the Begin government.
What will the United States do if Israel refuses to pull out of west Beirut? The President has uttered strong words, but he must have a plan for action if his demand is not met. Failing such a plan, his words will ring hollow and US credibility will suffer. So far, Mr. Reagan and his secretary of state have been careful not to threaten Israel with a suspension of military or economic aid, hoping that the ''promise of peace'' would create the pressure for new negotiations. But they cannot rule out a forceful response now.
How can the American nation ignore that the supply of US arms for purposes of self-defense has led to such gruesome slaying of innocent people?
What it is hoped will guide Mr. Begin is not, however, the persuasions of the US government or the outrage of the American people, but the moral conscience of Jews as they continue to seek a secure haven from persecution. Israel is at a watershed. It has invaded a neighboring country and seeks to impose its will on it. The result has been murder and destruction. Is that the ideal for which so many Jews have striven and suffered?
It is not too late for Israel to turn back from its willful course - and let the Lebanese reorder their own lives.