ASHINGTON, Sept. 21 ・When President Bush first sat down with his full cabinet after last week's terrorist attacks, he told them that nothing about their roles or charges as federal officials would ever be the same.

"I expect you to work hard on our agenda," Mr. Bush said, an almost obligatory nod to the various initiatives, like education reform and prescription drug coverage, that had consumed their attention before Sept. 11.

Then, a senior administration official said, Mr. Bush made it clear that all of that paled beside the war on terrorism that he planned to wage.

"This," he told them, "is the purpose of this administration."

That statement, which echoed and amplified others in the days after terrorists attacked the World Trade Center and the Pentagon, was apparently more than a succinct bit of White House cheerleading.

It was a window into what some of Mr. Bush's friends and advisers say is his own wholly transformed sense of himself and his presidency. He believes, they say, that he has come face to face with his life's mission, the task by which he will be defined and judged.

"He frequently says that we will be known to history by the way we approach this great cause," said one of his top White House aides, adding that Mr. Bush had made that statement to the religious leaders with whom he met in the White House just hours before his address to Congress on Thursday night.

One of the president's close acquaintances outside the White House said Mr. Bush clearly feels he has encountered his reason for being, a conviction informed and shaped by the president's own strain of Christianity.

"I think, in his frame, this is what God has asked him to do," the acquaintance said. "It offers him enormous clarity."

That is not something that Mr. Bush has always had. He often meandered through his life, occasionally ambled toward the presidency and exhibited a palpable ambivalence about his good political fortune along the way.

During the protracted, bitter denouement of the 2000 election, there were times when he seemed to shrink from the tension and recoil from the messiness, his eyes dazed, his shoulders slumped.

But many of the people around him say that now, facing an extraordinary crisis in his first year in office, he has acquired a kind of certainty that perhaps eluded him before. He is sure, they say, about what he should be doing. He is sure he cannot turn back.

Administration officials and others who have recently spoken with Mr. Bush differed in their assessments of how overtly religious his approach to his ・and the nation's ・current crisis is.

But they agreed that he was interpreting this juncture in grand, emphatic and even Manichaean terms, a perspective evident in his recent use of the word "crusade" and in his speech to Congress, in which he said that "this is civilization's fight," that freedom and fear were at war and that "God is not neutral between them."

People close to Mr. Bush attributed his poise in that speech, which he delivered without the stiffness or hyper-earnestness that characterized many previous turns in front of the teleprompter, to a heightened self-assurance about his priorities and a deepened determination about his responsibilities.

Karen P. Hughes, the counselor to the president, said that three nights before the speech, when he went over the first draft of it on the telephone with her, he said: "This is a defining moment. We have an opportunity to restructure the world toward freedom, and we have to get it right."

Senator Charles E. Schumer, Democrat of New York, who has met with Mr. Bush repeatedly since Sept. 11, said, "He has told me several times that he is staking his entire presidency on this ・that the mark of whether he's successful is whether he can succeed in his goal of wiping out terrorism."

Not everyone who has observed Mr. Bush's ardor and commitment views them as indisputably positive developments. Although the current moratorium on presidential criticism in the nation's capital prohibits most on-the-record carping, there is off-the-record concern, expressed not only by Democrats but also by some Republicans.

They fear that there is something headlong and immature in some of Mr. Bush's exhortations over the last few days. They wonder if he is making promises he cannot keep and threats he cannot back up.

They note it is impossible to know how ・and how much ・Mr. Bush has really changed, because efforts by the White House to control what gets said about him, and who says it, have been unusually aggressive.

Most of the people in a position to talk knowledgeably about Mr. Bush's emotions are not talking at all. Those who do talk have often sought the administration's permission, and they reel off the same adjectives, like focused and resolute, that White House spokesmen do.

Moreover, there are indications that Mr. Bush's nonchalant, jocular demeanor remains the same. In public, his off-the-cuff language still veers toward the colloquial. In private, say several Republicans close to the administration, he still slaps backs and uses baseball terminology, at one point promising that the terrorists were not "going to steal home on me."

He is not staying up all night, or even most of the night. He is taking time to play with his dogs and his cat. He is working out most days and arrived at a 6:30 p.m. speech rehearsal on Wednesday straight from a half-hour session on the treadmill.

But administration officials said that the president was investing certain duties, like Thursday night's speech, with extraordinary care. Ms. Hughes said that Mr. Bush had not been willing to schedule the address definitively until he was certain that he and his aides had nailed the speech, and she said that the event was not set in stone until Wednesday.

People who have visited the White House in recent days said there was a changed, charged atmosphere there. One of them, Mark McKinnon, a senior adviser to Mr. Bush's presidential campaign, said that the president obviously feels that the business at hand "is the country's destiny ・and his destiny."

Others who are close to the president said there was a discernible spiritual dimension to his thinking. A senior administration official recalled Mr. Bush's response on Thursday when one of the religious leaders said that Mr. Bush's leadership was part of God's plan.

"I accept the responsibility," the president said.

One of his close acquaintances said that Mr. Bush had essentially "begun a new life that is inextricably bound to Sept. 11 and all that it implies."

One implication, which he said he was sure that the president understood, was that from this moment forward Mr. Bush would be the despised enemy of violent extremists, and it might affect the tightness of the security around him even after his presidency.

For now, the acquaintance said, it was giving Mr. Bush the clearest, sharpest compass he had ever possessed.

"There's no question of what Bush's legacy will or won't be," he said. "He either beats this back ・or we lose."