MR. BEGIN'S LUNCHEON yesterday with the Senate Foreign Realtions Committe was perhaps the most important event on his Washington agenda. From old and close friends of his country, he encountered a markedly skeptical attitude of a sort to which Israeli prime ministers are not accustomed. The senators pressed Mr. Begin on his insistence on establishing new settlements inoccupied territories. They questioned his view that the call for return of occupied territories in U.N. Resolution 242 does not apply to the West Bank. Sen. Charles Percy (R-III.) later told reporters the settlements question "has divided Israel, divided the American Jewish community and caused an erosion of support for Israel."

Israelis have long felt that presidents, influenced by bureaucrats calling their attention to other factors, cannot be counted on to respcet Israel's view of its own security requirements; therefore, Israel must bring its political leverage to bear on the Congress. There lies the significance of Mr. Begin's luncheon. The Foreign Relations Committee, representing most Amercians on this issue, feels a great attachment to Israel. But it has deep misgivings about the Begin government's response to Anwar Sadat. Few would sell short the considerate steps Mr. Begin has taken. Many ask whether he has done what is necessary to ensure that the opportunity before Israel is not lost.

Doubtless the tempatation is great to attribute difficulties to Israel's political "enemies," or to look-to events like the latest Palestinian terrorist attack or the counterattack into Lebanon to win sympathy and divert criticism. Some Amercian Jews have even tried to set up one particular administration official, Zbigniew Brzezinski, as a scapegoat. There is an understandable reflex at work. But there is a deadly illusion, too. It is that Israel can, to its own advantage, avoid the difficult choices that lie between it and peace.

It is for Israelis themselves to face those fateful decisions. But it is for the American friends of Israel, including the American government, supporters on Capitol Hill and the American Jewish community, to offer their best judgement of what further policy changes from both Israelisand Arabs would best move the peace process along. This is not a question o f"imposing" a settlement, least of all of "abandoning" Israel, but of adding honest counsel to the very substantial patronage that will be forthcoming in any event.

Mr. Begin should be trying not merely to win backing for his policies but to understand what about them troubles so many Americans. If he can safely ignore Israel's "enemies," he must not ignore its friends.