DALLAS, Texas, May 19 -- Just before Pope John Paul II left for his recent trip to Greece, Bishop Joseph Fiorenza dined with the visibly frail pontiff, who turned 81 on Friday.

"He ate a hearty meal," said the bishop of the Diocese of Galveston-Houston.

"Physically, he's not in great shape. But he's a very strong man. He's got a lot of years ahead of him."

Now back in Rome after the historic six-day journey to Greece, Syria and Malta, the pope is calling the world's cardinals to the Vatican on Monday for a four-day gathering known as an extraordinary consistory.

The pope's age as well as health problems that leave him fatigued and limit his mobility are fueling speculation that he might resign. But many experts on the papacy say that's unlikely.

"That's not going to happen," said Father Thomas Reese, editor of the Jesuit magazine America and a Vatican expert. "He shows no indication that he's ready to step aside. His attitude is that he's pope right now and he's going to do everything he can to further the church."

The pope is planning a five-day trip to Ukraine next month. During this month's travels, he made gestures of reconciliation with the Greek Orthodox Church and with Muslims, becoming the first pope to pray in a mosque.

"He continues to surprise people," said Bishop Charles V. Grahmann of the Diocese of Dallas. "He just keeps breaking down all kinds of barriers that separate people. He's not afraid to do it, even when he's facing all kinds of resistance and criticism."

This is the sixth time since assuming the papacy in 1978 that the pope has called a consistory, and the first since appointing 37 new cardinals in January, which included three from the United States.

Expectations for the meeting are low among many U.S. observers, who say the pope is hoping to counter criticism that he doesn't collaborate sufficiently with ranking church leaders in setting church policy.

"He's looking for every opportunity to showcase his desire to consult because he's been vulnerable to criticisms that he isn't collegial," said the Rev. Richard McBrien, a theologian at the University of Notre Dame who has written extensively about the papacy.

Vatican spokesman Joaquin Navarro-Valls said that the closed-door sessions will be largely unstructured to allow genuine input from the cardinals. The pope has set no detailed agenda, though a starting point for discussion will be a recent apostolic letter, Novo Millennio Ineunte (At the Beginning of the New Millennium).

The lengthy letter is so broad and chock-full of issues that the Vatican has suggested topics for discussion. Among them: religious pluralism, the environment, globalization and economic injustice.

The Vatican's list didn't include any questions about papal authority or ecclesiastical power, which are points of discussion in the apostolic letter.

The Most Rev. Joseph Galante, coadjutor bishop of the Diocese of Dallas, said the potential agenda shows that despite the pope's age, he has a youthful mindset because he's looking to the future.

"He wants the church to be aware and well-positioned for this millennium," he said. "He wants to talk with the cardinals about how we can be more effective as a church. The pope is looking forward."

The pope's last consistory was seven years ago. Since then, he has appointed 60 new cardinals, bringing the total number to 183. Those under 80 will one day elect his successor from among their ranks, whenever the pope retires or dies.

"Sitting in that room will be the next pope," said Reese, who believes that thought will not be far from the minds of the cardinals.

"When one cardinal talks, the others will be listening, but also wondering in the back of their minds how that person would be as pope," he said. "Or, more importantly, when they're having coffee, they'll be sizing up how easy it is to chat with this or that cardinal. Does this guy listen? Is he easy to get along with?"

Although the Vatican is calling the meeting an advisory session, Reese said that it'll be difficult for the cardinals to offer any negative feedback to the pope, the man the church calls the Vicar of Christ.

"The problem the cardinals have in giving advice to the pope is that it infers that he's doing something wrong," he said. "So to avoid that, often at these meetings they spend half the time quoting the pope to himself."

McBrien agreed.

"It's not likely that the pope's going to get a real breadth of opinion and straight talk from a significant number of these men," he said. "They're his appointees. They're indebted to him. They already reflect his thinking."

The pope has been widely criticized for being less collaborative than his predecessors. During his reign, critics say, the papacy has become more top-down and authority more centralized than the model of power developed at the groundbreaking Second Vatican Council, which met from 1962 to 1965.

"This pope's style has been very authoritarian," said the Rev. Kilian McDonnell, a theologian at St. John's University in Collegeville, Minn., who's also a Vatican consultant.

"That raises questions about collegiality," he said. "Is it possible to have a strong papacy, strong leadership and still have input from national hierarchies and bishops? That should be possible, but it doesn't happen."

The pope previously called consistories in 1979, 1982, 1985, 1991 and 1994. Those gatherings focused on particular themes: church finances, abortion, religious sects and the church's jubilee year. The meetings have often ended without fanfare or any formal statements.

"It's too short of time to really to have anything substantial happen," McBrien said. "The document they're discussing covers a lot of territory. There's a lot of positive things in there, but you could use it as a basis of discussion for years."

Fiorenza, bishop in Houston, president of the National Conference of Catholic Bishops, said that theologians shouldn't be too quick to dismiss the pope's gathering.

"Despite his age and his infirmities, he has enormous courage and energy," he said. "He has a great interest in everything going on."