On Monday, April 21, President Carter was taking Cyrus R. Vance at his word.

America's rescue commando team had not yet begun its ill-fated mission, and Vance was very much still secretary of state. But the president, knowing of Vance's intention to resign if the mission was attempted, had already begun making a mental list of possible successors. And those closest to Carter say that the president's list had, from the outset, just two names.

Warren Christopher and Edmund Muskie.

One, the deputy secretary of state, highly regarded by the White House inner circle as a man of capability and loyalty, qualities whose importance to a White House cannot be underestimated and which are, in fact, probably presented here in reverse order.

The other a Democratic senator of national renown, a man whose political acceptability was probably broader than his contemporary knowledge of foreign policy (this being not necessarily a negative for this nomination) -- and a man who had quietly been increasingly supportive of Carter, especially during this current political season.

The president's deliberations about Vance's successor were interrupted by the hopeful initiation and sudden wrenching collapse of the rescue mission that was ensuring Vance's departure.

As the nation was conducting its post mortem of the startling raid that had failed, Carter returned to the question of who would be his new secretary of state. He needed no talent searches of lengthy lists of candidates. He is said to have consulted just two of his administration advisers: his chief of staff, Hamilton Jordan, and his vice president, Walter Mondale.

"Only two people around here knew about the president's choice on the new secretary of state last week," one senior White House official said.Among those specifically not in the know, he said, was Zbigniew Brzezinski, the president's national security adviser and Vance's policy protagonist.

There were arguments in favor of nominating each man. Christopher would provide clear continuity of policy and administration within the State Department; he had the respect of those within the department. Appointing him could forestall a mass exodus of those at the assistant and deputy assistant secretary level who might be inclined to follow Vance out the door over the principle of their opposition to the military rescue.

Muskie would bring to the job a national and even international reputation that could not be matched by Christopher, an accomplished but relatively colorless deputy. Muskie had great respect within political circles in general and Capitol Hill in particular.

Importantly, Muskie was not part of the Carter policymaking on Iran or any other international issue -- so his confirmation hearings could not be turned into an inquisition on the administration's decisions concerning Iran or afghanistan or anything else.

And finally, there was the matter of friendship, personal and political. The Carters and the Muskies had become friendly, in an official Washington way, with the Muskies dining with the Carters at the White House on occasion. Then there was the political friendship.

"Sen. Muskie has been very supportive of the president," said one senior adviser. And another added that "although Muskie did not publicly make an endorsement in the Carter versus Edward Kennedy contest of 1980, Muskie was helpful to the president in Maine."

While Mondale and Jordan were the only insiders who were in on the president's decision, there was one very in outsider who was at the White House last week as well. Charles Kirbo, Atlanta attorney who is the president's close friend, was talking to Carter last week.

"I don't like to comment on my dicussions with the president," is all that Kirbo would say yesterday when asked about his role in the selection process. But another senior official said he knows that Kirbo was aware of Muskie's selection over the weekend, at the very latest.

Kirbo had been involved before in deliberations concerning a new job for the veteran senior from Maine.

In the summer of 1976, in those preconvention days when it was clear that Carter had the Democratic presidential nomination won, Charles Kirbo came calling at the Senate office of Edmund Muskie. Kirbo was putting together a list of possible vice presidential choices for Carter. He told Muskie that he had come to consult with him as one of a group of distinguished Americans who might have some recommendations for the job.

Muskie, who had been Hubert H. Humphrey's vice presidential running mate in 1968, and who had sought the presidential nomination unsuccessfully in 1972, made it clear to Kirbo that he was irked that he was not under consideration for the number two spot on Carter's ticket in 1976.

Kirbo dutifully added Muskie to the list, and in July Muskie was the first candidate summoned to Plains, Ga., for what amounted to a job application interview with Carter.

Muskie had come hurrying anxiously to the job interview, flying all night from Maine and landing on the grass strip in Plains at 2:30 a.m. He conferred with Carter that night and the next day. Eventually he was a semi-finalist, along with a younger senator, Mondale of Minnesota. Carter is said to have been concerned about reports of Muskie's hair-triggered temper and that he had been known to be unnecessarily harsh with his staff.

Carter opted for Mondale. It was, his advisers say, a matter of chemistry.

On Sunday, April 27, 1980, Carter opted for Muskie. It was, his advisers say, a matter of compatibility.

Carter telephoned the Maine Democrat to tell him of his decision and urge him to accept. Muskie, who was in Nashville, told the president that he would think it over.

On Monday, Mondale telephoned Muskie and urged him strongly to take the job. And that night, Muskie went to the White House and told Carter that he would leave the Senate to serve as secretary of state.

EPILOGUE: One of the president's most senior advisers is effusive about the way things have turned out. "This is the best of all possible worlds," he said over the telephone yesterday. "We have Muskie and Christopher at State now. A respected national figure and a good manager widely respected in the department and on the Hill. It has worked out very well."

No sooner did he hang up the phone than it rang again. An aide to Christopher was calling to spread the word that Christopher was not going to stay as deputy of State very long after all. "He is committed to a solid transition," the aide said. ". . . He's not jumping ship quick . . . Christopher thinks Muskie ought to be able to pick his own team."