Education Secretary William J. Bennett is just back from a spin through Europe, where he met at length with British Prime Minister Margaret Thatcher and visited with Pope John Paul II. He stopped by the Berlin Wall, lectured a West Berlin audience on the need to defend against totalitarian threats, and, upon his return to American soil, called in reporters to recount his travels.

It was not the only unfamiliar turf the nation's education leader has trod upon in recent months. He has ventured into public health policy, making himself something of a conservative spokesman for the Reagan administration by issuing a controversial call for widespread, routine AIDS-virus testing.

He has stepped into Republican politics, delivering a keynote address at a party gathering in March, just a year after his conversion from Democratic ranks. And he has become one of the most vocal Reagan stalwarts, repeatedly scolding Republicans for deserting their president in the heat of the Iran-contra affair.

Recently, in an Evans and Novak broadcast on Cable News Network, he lashed out at members of the Iran-contra investigating committees, saying they were set on destroying the Reagan presidency and labeling their motivations "venal, petty and political."

Bennett's departure from the traditional bounds of his post has separated him from his predecessors and transformed the secretary of this relatively small, low-level agency into one of the most visible Cabinet members. In the process, he has been faulted for what critics see as inappropriate and opportunistic self-promotion. But others like his willingness to take controversial stands.

The strongest reaction was prompted by the secretary's May speech calling for routine AIDS tests for hospital patients, immigrants, prisoners and couples applying for marriage licenses.

"I found it amazing that the secretary of education, without a health background or expertise or authority, would try to push all the health experts within the Reagan administration aside and advocate policies with this most serious epidemic we've ever faced," said Rep. Henry A. Waxman (D-Calif.), who has been critical of the administration's response to acquired immune deficiency syndrome.

"He seems to have ambitions far beyond being secretary of education. He wants to be a political spokesman," said Waxman. "Maybe he even fancies himself being on the Republican ticket some day."

Bennett says he may run for office in the distant future, but has no immediate political ambitions and dismisses that as his motivation.

"What's the law that says the secretary of education can't talk about other things?" he asked in a recent interview. "I want to stimulate discussion, and, boy, we've had great success . . . . As long as people are going to listen and debate, like it or not, I'll keep doing it."

Since he took over from former education secretary T.H. Bell in 1985, Bennett has transformed the office, which oversees a quiet department with comparatively limited programmatic responsibilities. His controversial utterances have not only expanded his visibility outside the traditional agency realm, but have alienated much of its natural constituency, the education community.

Unlike Bell, whom educators regarded as a colleague and advocate, Bennett has been seen as an adversarial outsider who refused to go up against administration budget-cutters. He has brought in a number of conservative advisers and, while arguing for a limited federal role in education, has used the platform to promote his views on a host of issues.

"What does bother some of us is exactly the same thing that bothers us about his pronouncements within education: they have a certain hit-and-run character," said Stanford University President Donald Kennedy. "There is something often thought-provoking, often controversial, but it isn't revisited in a serious, thoughtful way."

No one has accused the secretary of neglecting his department; in fact his success in boosting education on the national agenda is widely acknowledged. But Bennett has made it clear he doesn't mind creating tension with the education establishment, and he has taken numerous opportunities both to criticize educational institutions and make his mark in other circles.

Last fall, he made an unusual entry into partisan politics when he delivered an untempered blast at Republican presidential aspirant Marion G. (Pat) Robertson, labeling as "invidious sectarianism" Robertson's suggestion that Christians "feel more strongly" than others about love of country, God and the traditional family.

In the days after the first Iran-contra revelations, he jumped at the chance to go on television to reproach those who were distancing themselves from the president. And he recently blasted Democrats for their early opposition to Supreme Court nominee Robert H. Bork, a poker buddy.

He made a public show of his loyalties by meeting Assistant Secretary of State Elliott Abrams for lunch at the White House when Abrams was under intense criticism for his role in the Iran-contra affair.

In addition to his call for widespread AIDS testing -- he supports testing everyone between the ages of 15 and 60 who is undergoing a blood test for any reason -- Bennett said in June that prisoners infected with the AIDS virus should be isolated and that officials should consider not releasing those who threaten to spread the infection.

This foray into the public health arena has inspired as much confusion as controversy.

"Secretary Bennett is not a public health official, but he seems to be in a position of being allowed by the administration to propose substantive health policy," said Kristine M. Gebbie, administrator of Oregon's health division and head of the AIDS Committee of the Association of State and Territorial Health Officers. "I doubt people in the education community would have the secretary of health and human services, Mr. {Otis R.} Bowen, proposing major education policy."

At a congressional hearing on AIDS this spring, Waxman asked Surgeon General C. Everett Koop, who has opposed mandatory testing, whether he or Bennett was the spokesman for the administration.

"That is a difficult question, sir," replied Koop.

And in an interview last week, Waxman added: "I think of {Bennett's} involvement in this AIDS issue as much more a political statement by him to appeal to the political right wing than anything else."

Bennett defends his involvement in the AIDS-testing debate, saying, "We've done our homework" and arguing that the matter is integrally related to education.

"As a member of the Domestic Policy Council, I was asked to think about it," he said. "People say {AIDS} has to be addressed by education. How can people then object to the secretary of education talking about it?"

Robert R. Redfield, a prominent Army AIDS researcher, played down the differences on the testing issue between Bennett and Koop, who has emphasized the need for explicit AIDS education and cautioned against mandatory testing. Redfield praised Bennett's success in focusing attention on the alternatives.

"Any leader at a secretarial level can't be so parochial as to look just at their own little niche," said Redfield. "I embrace the position expressed by Secretary Bennett . . . . He's done more to move that argument forward than anybody else in the current administration."

In June, President Reagan called for greatly expanded AIDS testing, adopting a position close to that advocated by Bennett.

Albert Shanker, president of the American Federation of Teachers, doesn't fault Bennett for his involvement in non-education questions, but says the secretary has fallen down at his traditional role of representing the nation's education community.

"He's done a very good job of representing his own and the administration's point of view to the country, but I think he's done a poor job of trying to get the administration to understand the problems of schools," Shanker said.

Examples of Bennett's combative style have become well known in education circles: he has sharply criticized colleges for rising costs, mediocrity and permissiveness toward drug use on campus. He has taken on the teacher unions and others in the education establishment for slowing the education reform movement. And he has, until recently, proposed sharp reductions in the federal education budget.

Stanford President Kennedy compares the secretary's statements on AIDS-testing policy with another controversial comment -- Bennett's charge that college tuition has gone up because of the availability of federal student financial aid.

"I don't think he's stayed with {the AIDS} issue and provided a thoughtful second-round solution," Kennedy said. "Rather he's left it and gone on to different things. It's exactly the same on tuition charges." Despite subsequent studies indicating the secretary is mistaken on the tuition matter, Kennedy said, "Bennett will not return to that issue and discuss it at a thoughtful depth."

While Bennett's recent behavior is perceived by many to be politically motivated, he argues that it is precisely his lack of immediate political ambition that allows him to speak out on issues so forcefully.

"Isn't it a cardinal rule for the 36 candidates now running to be careful?" he asked. "I want to do this job, finish, maybe get into politics later . . . a run at the governor's job someday."

He doesn't know where he would launch such a campaign, but says he will keep his hand in party politics. In the meantime, he likes the headlines and his heightened profile.

In describing his recent meeting with Thatcher, he reflected: "When you sit there with the prime minister of England . . . and she's holding your {department's} book and speeches in her lap and says, 'This is excellent,' . . . this tends to give you confidence."

While Bennett's flamboyant style can be admired on some counts, said University of Miami president Edward T. Foote II, it also has been counterproductive. "There is so much acrimony floating around, we don't get to the merits" of debating education, he said.

"It's not unhealthy for somebody of national visibility to raise questions . . . to move outside the tight margins of the office," Foote said. "Where it gets tricky is moving outside his area of expertise . . . . We've got plenty on our plate with just education without stepping off into other fields."