He met Saturday with the archbishop of Canterbury, in a meeting that was clouded by deepening divisions over homosexuality.
Pope John Paul II warned the archbishop of Canterbury of "serious difficulties" in efforts to unify the Roman Catholic and Anglican churches, pressing the Vatican's case against the U.S. Episcopal Church's election of its first openly gay bishop.
The visit was the first to the Vatican by Archbishop Rowan Williams since his installation in February and came two weeks before an emergency meeting of the Anglican Communion's 38 primates to discuss the controversial election of the openly gay V. Gene Robinson as bishop of New Hampshire.
"There's a significant possibility that the Anglican communion itself may split apart," said Dave Anderson, an organizer of a meeting of conservative Episcopalians who plan on petitioning Robinson's appointment, according to CBS News' Randall Pinkston.
"The issue of homosexuality is very clearly a moral issue, and certainly, raising a person to high levels of leadership compounds the problem," Anderson added.
Williams said the pope's view about the risks to Catholic-Anglican relations "weighs very heavily" on his Church going into the emergency meeting.
"I hope none of what has been achieved in all these years will be lost," he said.
The Vatican recently issued a new broad condemnation of homosexuality that urged Catholics and non-Catholics alike to unite to stem a trend toward granting legal recognition to same-sex unions. The document called homosexuality a "troubling moral and social phenomenon" and repeated the Vatican's position that homosexual acts were "intrinsically disordered."
Neither the pope nor Williams made direct mention of the issue, but it was clear the pope was referring to it as the two men exchanged speeches following 15 minutes of private talks.
"As we give thanks for the progress that has already been made we must also recognize that new and serious difficulties have arisen on the path to unity," John Paul said, sitting side-by-side with Williams. "These difficulties are not all of a merely disciplinary nature; some extend to essential matters of faith and morals."
With increasing secularism in the world, the pope continued, "the Church must ensure that the deposit of faith is proclaimed in its integrity and preserved from erroneous and misguided interpretations."
The frail 83-year-old pope, who suffers from Parkinson's disease, read his speech with difficulty but completed the entire text.
Since Williams began his talks Friday, both sides stressed the importance of theological efforts to overcome what John Paul called "the sad division" between the Churches.
The Anglicans split from Rome more than four centuries ago when King Henry VIII bolted in 1534 over the pope's refusal to grant him an annulment.
The Episcopal Church is the U.S. branch of the 77 million-member global Anglican Communion.
"Until recently, one could state with relative confidence that Catholics and Anglicans shared the same moral principles regarding human sexuality," said Cardinal Walter Kasper, in charge of the Vatican's relations with other Christians, and who sat next to Williams at a news conference. "We hope that it will remain so, for the world today needs our common witness."
Williams, well aware of the Vatican's position, concentrated on efforts to overcome theological divisions between the churches.
"I am glad to reaffirm my commitment to the full, visible unity of the Church of Christ," he said.
After the Episcopal Church confirmed Robinson as bishop in August, several Anglican bishops overseas threatened to sever ties with the Americans.
Conservative Episcopalians within the United States also threatened to break from their denomination. U.S. conservatives will gather in Dallas next week to discuss their next move.
Vatican-Anglican relations were already strained in 1992, when the Church of England decided to ordain women. At the time, the Vatican, which reserves the priesthood for men, called the decision a "new and grave obstacle to the entire process of reconciliation with the Catholic Church."