In his opening words about Bert Lance, Jimmy Carter called him "my friend." But, as he quickly made clear, they were much more than that. Like the biblical Jonathan and David, they were inseparable, brothers under the skin.

Yesterday, Carter took on a brother's role for his fallen friend. It was Jimmy who had caused Bert anguish.

"It was I who insisted that Bert agree to sell his substantial holdings in bank stock," he said. "Had he stayed there is a selfish fashion and enriched himself and his own family financially, I'm sure he would have been spared any allegations of impropriety. But he wanted to come to Washington and serve his government."

Then he added, in underscoring his theme:

"Because I asked him to, and he did."

His insistence on "extraordinary standards" in government, his promises during his campaign of something better, his pledges during his inauguration - all these had made it difficult for Bert Lance.

Like John F. Kennedy and the Bay of Pigs, Carter met his first presidential crisis by putting the blame on himself.

"I am the responsible officer of this government," Kennedy had said in attempting to put to rest any scapegoating about that disaster.

"A lot of the problem has been brought on Bert Lance by me," Carter said yesterday.

Kennedy was astounded to find that his popularity, as measured by the Gallup Poll, climbed to its hightest point after that crisis. What the public reaction will be to Carter's final resolution of the Lance case is, at this writing unclear. But under the circumstances, it was probably as effective a performance as could be given.

There was none of the mawkishness that marked earlier occasions when Presidents have found themselves forced to preside over the disposal of key friends and aides - none of the implicit hints that the President himself really thought his assistant had betrayed him.

But neither, in all of the President's words, was there any concession that Lance had done anything wrong nor that he had in any way sullied the reputation of the young Carter administration.

The President yesterday maintained, from beginning to end, that Lance was a man of honor and that he had exonerated himself. As Carter said, "He (Lance) has told the truth, but I think he proved that our system of government works because when he was given a chance to testify on his own behalf, he was able to clear his name."

Carter continued that type of defense during the 33 minutes of questioning from the White House press corps. The FBI report about Lance, containing some 100 interviews, would demonstrate that, Carter said.

"All of these analyses of bert Lance's character and ability were good and favorable," the President stressed.

Neither would be concede that "any mistake was made" in selecting Lance: ". . . He was qualified then, I think he's qualified now . . ." The same was true of recent allegations that Lance might have concealed something from the President and his staff. That just wasn't so, in Carter's eyes.

Aside from the political questions raised at yesterday's press conference and puzzle about how much damage will result from the Lance affair, perhaps the single most revealing aspect was the President's description of his relationship with Lance.

In an earlier time of trouble, under different circumstances. Dwight D. Eisenhower had said of his closest aide, Sherman Adams. "I need him" Jimmy Carter went beyond that yesterday. What he was expressing was how extraordinarily close he and Lance have been.

they were closer than partners even. As the President put it, "Bert Lance and I communicate without embarrassment, without constraint and without evasion of issues."

Indeed, Carter virtually said that Lance will be irreplacable.

"I don't think that there is any way that I could find anyone to replace Bert Lance that would be in my judgment as competent, as strong, as decent and as close to me as a friend and adviser as he has been," he said at one point.

And later, he referred to their "special relationship, one that "transcended official responsibilities or duties or even governmental service over the last six or seven years."

The hardest question, one that came at the President in a number of forms yesterday, was how seriously this incident has affected his presidency. On that, Jimmy Carter was uncertain.

"Whether my own credibility has been damaged, I can't say," he remarked.

His guess, he went on, was that "to some degree" it would be damaging, "somewhat."

That was undoubtedly correct, but the most difficult fact remained: after only eight months, another presidency has been put through a test involving its credibility and character. The question was over the extent of the damage.