THE ISRAELI political system has been massaging the findings of the Beirut massacre inquiry, with disappointing results. It is a good thing for Ariel Sharon to have been removed from the defense ministry and from direct control of the country's armed forces. No doubt his designated replacement, Ambassador Moshe Arens, intends to be defense minister in fact as well as name. But Mr. Sharon stays in the government and in a position to exercise some policy influence and continue his quest to succeed Prime Minister Menachem Begin.

For a while, many Israelis and others had hoped Mr. Begin would rise above political routine and accept the inquiry report in its broad spirit of political renewal. Mr. Begin chose instead to flatten the renewal, accepting the report in a minimal legalistic interpretation. He cut Mr. Sharon loose as defense chief only with the greatest reluctance and remains in his evident personal as well as political debt. After all the expectations stirred by the inquiry and the inquiry report, the political outcome has the look of a long climb for a short slide.

This may be a time to come to grips with an important fact: the Begin government will not soon or easily be beaten at the polls or swept aside by a tide of public opinion or a collapse of nerve. It is not for all time, but its durability is attested by the storms it has stirred and survived. That government's political opposition, moreover, is every politician's dream.

The practical significance of all this for the Reagan administration is that it must deal with the Begin government as it is--democratic and independent-minded--and not figure that some successor government is going to arrive on the scene and somehow make things easier for the United States. With or without such an event, it is only to be expected that the Begin government will continue to pursue Israeli interests--where these diverge from American interests--at least to the extent that it believes Americans will go along with its independent policy.

Quite obviously, the torment touched off in Israel by the Phalangist massacre in Beirut was not enough to erase the gains that the Israeli public otherwise finds were made in the Lebanese operation. That leaves as the only conceivable source of a political turn in Israel a chain of decisions by Jordan and the Palestinians presenting Israelis with a live negotiating partner in the West Bank.