Secretary of State George P. Shultz, declaring that "we think peace is a winner," assured Congress yesterday that President Reagan is determined not to let his Middle East peace initiative be sidetracked and intends instead to make it the basis for a settlement of the Arab-Israeli conflict.

Testifying before the House Foreign Affairs Committee for the first time since assuming office, Shultz informally launched the administration's campaign to build broad support for Reagan's initiative and to serve notice on the other countries involved that the president will not be deterred by rebuffs such as Israel's categorical rejection of the plan.

Although Shultz did not allude to it during 2 1/2 hours of testimony before the largely sympathetic committee, administration officials privately expressed elation over the decision Wednesday by the largest American-Jewish service organization, B'nai B'rith International, to publicly praise the broad outlines of Reagan's plan and call it "worthy of consideration."

In a speech here last night, Maynard Wishner, president of the American Jewish Committee, another organization influential in Jewish communal affairs, gave the initiative a similar, qualified endorsement. Wishner said Reagan's plan "deserves thoughtful and thorough consideration" and called it "a reasonable approach to be dealt with on its merits."

Shultz's testimony contained no startling new revelations about the proposal, which calls for Israel to halt new settlements in the occupied West Bank and Gaza Strip and for those territories eventually to gain self-rule "in association with" Jordan. In exchange, the Reagan plan envisions the Arab world giving Israel recognition and guarantees of its right to exist within secure boundaries.

All major aspects of the president's plan have been disclosed publicly, and there have been no secret understandings or approaches to Arab governments, Shultz said. He asserted, "There are no big, secret things going on."

But his testimony was noteworthy because of what it revealed about the administration's strategy for pursuing its initiative and manuevering the other countries involved, particularly Israel, toward the bargaining table.

After stressing that the administration plans to rely solely on persuasion and does not intend to use military or economic aid to pressure Israel, Shultz declared: "We have put positions forward that we think are reasonable and constructive and that above all are consistent with the idea of peace in the area . . . . People are reacting to it. We'll work with the situation, and we'll just have to see. But we think peace is a winner."

Underlying his words is a strategy that administration sources privately describe as involving a dogged, slogging process, likely to take several months. It is designed to chip away at Israeli Prime Minister Menachem Begin's determination to incorporate the West Bank into Israel and bring him to a more flexible negotiating position.

A key element in this strategy is the premise that, in the wake of the Palestine Liberation Organization's expulsion from Lebanon and the dispersal of its military forces, the moderate Arab governments gradually will move away from their insistence that the PLO is the only legitimate spokesman for the Palestinian cause and give Jordan's King Hussein a green light to join Israel and Egypt in negotiating the future of the occupied territories.

The administration, working on the assumption that Hussein eventually will cooperate, has focused on trying to moderate Begin's hard-line policies by mustering strong support from Congress, the American public and other governments for the president's initiative.

As described by the sources, the hope is that the Israeli public, realizing it is becoming isolated from the United States, will then be forced into a major national debate, leading the Jewish state to reconsider the plan that Shultz described yesterday as "trading land for security."

A key aspect of this strategy aims at bringing significant elements of the American Jewish community, where there is considerable private disagreement with Begin's tough policies, into open backing of the Reagan initiative. That is causing some clear divisions among Jewish leaders and organizations here, with some cleaving to their traditional path of unwavering support for the Israeli government and some saying publicly that the president's plan deserves to be pursued.

That was the line taken by Wishner of the American Jewish Committee in his speech last night. He agreed, as have the B'nai B'rith and other Jewish spokesmen sympathetic to the president's ideas, that the safeguarding of Israel's security is paramount and that reciprocal steps leading to recognition of Israel must be taken by Jordan and other Arab states.

But he stressed that the peace initiative, "judged as a whole," is "a reasonable approach to be dealt with on its merits." Wishner also noted that the AJC twice has called for "pause and restraint" in Israel's settlements policy, and he added that if Jordan joins the peace process, "I would again be willing to call upon Israel for such pause and restraint."

In his testimony, Shultz conceded that in presenting the plan to Israel and Arab governments the United States had included a series of "talking points" that were not all spelled out publicly when the president revealed his initiative on Sept. 1. In summarizing the administration's position yesterday, he went over all the positions covered by these "talking points."

Specifically, Shultz said, the United States will support: full autonomy for the Palestinian inhabitants of the West Bank and Gaza; the economic, social and cultural ties of these territories with Jordan; permitting Palestinians in East Jerusalem to vote for the self-governing authority in the territories; Palestinian responsibility for their own internal security, and a freeze on Jewish settlements.

Conversely, he added, the United States will oppose attempts to incorporate the West Bank and Gaza into Israel or to make them an independent Palestinian state. It also will oppose dismantlement of existing Jewish settlements in these areas and any actions that pose a legitimate threat to Israel's security.