Warren Christopher has been the Carter administration's favorite secret weapon for years. Now the secret is out.

President Carter has surprised listeners in recent days with private and public remarks describing the deputy secretary of state as the single best public servant he encountered during his presidency. But this admiration has existed at the White House since the spring of 1978, when Christopher saved the Panama Canal treaties.

Since then Christopher has been a one-man fire department for the president. If the bargain with Iran for the release of American hostages sticks, Christopher will have won his grandest laurels at the last possible moment of the Carter administration.

Carter's esteem for Christopher is matched by most of his senior aides and also by the senior officials at the State Department. But Christopher himself has not turned their high regard into a matching public reputation. On the contrary, he has transformed the adjectives "quiet" and "self-effacing" into a way of life. This is about as common in Washington as cool, breezy August afternoons.

Warren Minor Christopher is now 55. He grew up in South Dakota, the son of a banker who went under during the Depression and moved his family to California. Christopher had a brilliant career as a Stanford law student, finishing a editor-in-chief of the law review, then came to Washington as a clerk for Supreme Court Justice William O. Douglas. A Supreme Court clerkship is an elevator that swoops the passenger into the upper reaches of American life, and Warren Christopher happily took the ride.

He went to work for one of Los Angeles' biggest and most distinguished law firms, O'Melveny & Myers. His diligence, tact and intelligence quickly shone, according to legal associates. He began to dabble in Democratic Party politics, and helped out in the federal study of the causes of the Watts riot. Later, he came to Washington, serving as deputy attorney general in the Johnson administration.

Cyrus Vance picked Christopher as his deputy, with the ardent support of Lloyd Cutler, a friend of both men and himself a key player in the Iranian negotiations as President Carter's legal adviser. Christopher had little experience in foreign affairs, a fact that raised some eyebrows when he was named to the State Department job, and which did slow him up somewhat in the first months of the Carter administration.

But Christopher was a quick student, according to his colleagues, and by the time the Senate began to debate the controversial Panama Canal treaties in early 1978 he was ready to play an important part. As it turned out, he played the crucial role for the administration.

"Chris turned out to be the chief negotiator in the Senate -- with our side [treaty supporters] and with the other side," recalled Dan Tate, the White House's chief lobbyist in the Senate. Tate and other officials say that Christopher has a keen political sense, better than any other senior official in the State Department.

It fell on Christopher first to placate the last key group of swing senators -- those who demanded the "DeConcini reservation" reserving as the U.S. right to protect the canal as the price of their support -- and then to persuade the Panamanians to accept this reservation, which is named for Dennis DeConcini (D-Ariz.).

Christopher sensed that there was no way around the DeConcini proposal. He worked to make it as painless as possible, but he counseled Carter to accept it, a move that provoked a lot of criticism. When Panama's "maximum leader," Gen. Omar Torrijos, then made threatening noises about refusing to accept the DeConcini reservation, Christopher plotted the administration's reply.

Christopher never has received public credit for his successful defense of the treaties. First he gave DeConcini and like-minded senators what they wanted, then he looked for a way to placate Torrijos. He advised one, using the services of Washington attorney William Rogers, a former State Department official and friend of the Panamanians, that ended up pleasing all parties.

"He masterminded it," was the way Assistant Secretary of State Richard Moose put it yesterday.

Christopher's success with one hard job earned him a succession of others. He became the quarterback for the administration's most controversial foreign policy initiatives: lifting the Turkish arms embargo, selling advanced fighter aircraft to Saudi Arabia and Egypt, ending official state-to-state relations with Taiwan, and negotiating the deal for the release of the hostages in Iran.

The tactics for handling these problems were often the subject of discussion at an important regular meeting that Christopher ran in the State Department, and which never received any publicity. That was the deputy secretary's "Monday Lunch," attended by the political appointees in the top level of the department. It was one of the best regular meetings in town, according to Douglas J. Bennet, administrator of the Agency for International Development.

Christopher was deeply disappointed when Carter passed him over to pick Sen. Edmund Muskie as secretary after Vance resigned. Carter privately has been apologetic about his choice, explaining that he needed a prominent statesman to take Vance's place. Carter persuaded Christopher to stay on under Muskie, who quickly came to depend on his ubiquitous number two man -- as Christopher's key role in the Iranian negotiations revealed.

Friends expect Christopher to go back to his law practice in Los Angeles when he winds up his official duties.

"But I can't imagine he won't be back in Washington some day," one colleague observed yesterday.