SAN FRANCISCO, SEPT. 17 -- Pope John Paul II's strong words Wednesday for Roman Catholics who dissent from church teachings are likely to alienate or drive away many of the faithful, several leading Catholic thinkers said today.

They said they are particularly concerned about Catholic theologians, ethicists, doctors and scientists who are trying to advance society's understanding of areas in which the Vatican is most unbending -- sexual morals and reproductive technology.

John Paul's statement to the bishops, much blunter than earlier speeches to other constituencies, "relegates the Catholic thinker and scientist to the sideline," said Warren T. Reich, professor of bioethics at Georgetown University, a Catholic institution. "There are many of us who don't care to be there."

The pope's speech came in response to a gentle plea by four senior U.S. bishops that he try to understand the peculiar independence of American Catholics. He is likely to hear a similar message Friday when he meets with with representatives of the laity in what may be one of the most crucial encounters of his 10-day tour.

John Paul told the bishops that dissent from church teachings on divorce, remarriage, abortion and birth control is incompatible with being "a good Catholic." He said it is a "grave error" to think that such dissent "is no obstacle to reception of the sacraments."

Although he said university theologians have the right to a "legitimate freedom of inquiry," he added that the "title Catholic theologian expresses . . . a responsibility to the service of the community of faith, subject to the authority of the pastors of the church."

"In particular," he told the bishops, "your dialogue will seek to show the inacceptability of dissent and confrontation as a policy and method in the area of church teaching." While he has reiterated his positions on moral issues in meetings with other Catholic constituencies on this trip, they generally have found him encouraging and compassionate.

"The tenor of his remarks was more low-key than I had expected," said Sister Alice Gallin of the Association of Catholic Colleges and Universities, which met with the pope Sunday. "I felt affirmed."

"What I heard {last Thursday} was that there is a dialogue going on," said the Rev. Richard Hynes, president of the National Federation of Priests' Councils.

On behalf of about 700 priests, the Rev. Frank McNulty urged the church to consider important changes in the priesthood. Hynes said he felt that the pope largely rejected McNulty's recommendation, but "received it without harshness. That in itself means change is possible."

After news reports today of the bishop's meeting, Catholic representatives were less sanguine, their concerns centering on academics who teach doctrine and morality and lay Catholics who must follow their lead.

"Not every issue demands the same assent," said the Rev. William Rewak, president of Santa Clara University in Santa Clara, Calif. In his view, there is no room for disagreement on the church's opposition to abortion, for example, but he believes it is possible to favor women in the priesthood and remain "a good Catholic."

Reich, the Georgetown bioethicist, said the pope justifiably wants Catholics to share a consistent moral code. But he said that unless the elements of that code are continually tested and, if necessary, revised, Catholics may reject it completely.

Church teachings on sexuality "rest solely on the moral convictions of another era," Reich said. The Vatican has made little attempt, he said, to acknowledge that moral values can change with time and technology.

The pope observed sternly Wednesday the tendency of American Catholics "to be selective in their adherence to the church's moral teachings." He said they should bring their education and liberty to bear in bringing the Gospel to "the whole realm of thought and artistic creativity, to the various professions, to family life and to society in general."

Reich cited in vitro fertilization, a procedure sometimes used by infertile couples in which egg and sperm are joined in a laboratory and inserted into the mother's womb. In a document last spring, the Vatican rejected such procedures without considering the moral value of the closeness that might be fostered in a family with the birth of a child, Reich said.

Reich, who works closely with medical researchers and doctors at Georgetown University, said he is particularly concerned that Catholic institutions are precisely the schools where such new procedures should be encouraged, because they could be developed, tested and carried out in an environment where moral considerations are paramount.

"It's a tragedy that these moral resources are not engaged in this dialogue," he said.

According to sources at Washington, D.C., hospitals, some Catholic researchers have moved any work that might be controversial to other institutions. The most advanced work in reproductive technology is being done at non-Catholic universities, medical authorities said.

Education sources cite a similar "brain drain" at Catholic schools of theologians who teach church doctrine to the next generation. Those who stay are cautious and may become more so, according to Anthony Timbusco, a professor of moral theology at Georgetown.

Theologians were first alarmed when the Vatican recently barred the Rev. Charles E. Curran from teaching moral theology at Catholic University because he dissented on matters of sexual morality. Now "we live under the threat that we, too, might be silenced," Timbusco said.