President Anwar Sadat's glorious reception back in Cairo Monday conclusively proved that he had read the Egyptian mood correctly, but the real test has not yet come: how much Menahem Begin finally will give Sadat in return for his courageous visit.

Before arriving in Jerusalem last week, Sadat was quietly laying plans to appear in triumph before his parliament at the earliest possible moment to reveal to his own citizens what had led him to Jerusalem in one of history's most celebrated peace missions. When he actually got back to Cairo, he set the date for Saturday, Nov. 26, the sixth day after his return.

That's because Sadat came home Monday with little more than the echo of emotional words of praise and the memory of thousands of tear-stained Israeli faces that paid him homage. Beyond the flags and the tributes, Prime Minister Begin apparently offered little of substance. He carefully balanced Sadat's demand for the return of Arab territory ("our land is sacred") with exactly the same words to explain Israel's decision not to return to the 1967 borders. "May I say," Begin told Sadat, "our land is sacred."

Just before the visit to Jerusalem, Mideast experts here feared that unless Sadat made quick, substantive gains from his journey he would face not only escalation of anti-Sadat emotions throughout the militant Arab world; he also would be courting disaster at home! Economic grievances, particularly the inflation of food prices and monstrous unemployment among young college graduates, are ready-made for political exploitation against Sadat.

But Sadat gained elbow room by emphasizing in his talks with Begin, problems of all Arab nations with grievances against Israel that underemphasized both Egypt and a separate Egyptian-Israeli peace. This approach made possible the euphoric welcome for Sadat in Cairo.

More important were breaks in the ranks of Arab states that had been keeping discreet silence. Jordan, now politically tied to Syria more closely than ever before, broke its silence with a modest pat on Sadat's back for daring to go to Israel despite anti-Sadat fury in Damascus.

Sadat's success so far in maintaining an image of great popularity at home and limiting anti-Sadat damage in the Arab world could disappear overnight if he has to acknowledge to his parliament that Israel gave him nothing in return. That risk, and the reasons for Sadat's taking it in the decisions that led him to Jerusalem, puzzle both American officials and Arab diplomats here.

The complete answer may not be known for many months. But the prevailing view is that Sadat had just about given up on the Carter administration's efforts to extract real concessions from Prime Minister Begin. He was, in short, at the end of his patience. Nor was Sadat, the Egyptian patriot who ordered Russian military advisers out of his country in 1972, pleased at the new prominence given Moscow's re-entry into the Middle East in the Oct. 1 U.S.-Soviet joint statement.

This go-it-alone mood of frustration and worry led Sadat to make his bold declaration of independence at his farewell Jerusalem press conference on Monday.

Pointing to his "strained relations" with Moscow, Sadat said he worried that anti-Egypt Soviet conduct "could be adopted in Geneva," where the Russians are co-chairmen with the United States. "Whenever we, the [Arab and Israeli] parties concerned, reach an agreement, no one, big power or small power, can prevent us from fulfilling it inasmuch as we have agreed to it."

Carter administration officials say this simply underlined Sadat's persistent skepticism of returning to a Geneva conference not carefully prepared in advance. But they may be underestimating Sadat. Kept in the dark every step of Sadat's course to Jerusalem, the Carter administration has demonstrated a conspicuous ignorance about the president of Egypt.

Thus, the harsher view of Sadat's apparent declaration of independence may be the correct one: Without disowning a reconvened Geneva conference, Sadat's trip to Jerusalem declared Egypt's independence not only from his Arab partners, but from his American friends as well. In Cairo's view, the United States has repeatedly proven its inability to deliver on promises to Egypt when Israel stands in the way.

To gain from this declaration of independence, however, Sadat must sooner or later be able to show his country why his journey to Jerusalem was undertaken. In short, he must show how it will help Egypt - plus Syria, Jordan and Lebanon - find the elusive peace he has promised, and that takes some help from Begin.