DENVER -- When Pope John Paul II flies here this afternoon and is greeted by President Clinton, it will be the first meeting between this uncompromising pope preaching changelessness and a politician for whom change is a goal and compromise is a way of life.
The differences between them -- age, religion, social values and methods -- are differences between an Old World Vatican and the New World, including its Roman Catholics. The pope thinks that this nation is obsessed with quick fixes, sexual gratification and "consumerism."
Now 73 and stooped, the white- robed religious leader shows no sign of letting up on his campaign to transform American culture.
On the 60th international trip, the third to the United States, of his 15-year papacy, Pope John Paul is expected to repeat the traditionalist themes of his cross-country tours in 1979 and 1987.
But this week, at scattered events in the Denver area culminating with an outdoor Mass in a state park on Sunday, his sermons and speeches will be directed particularly at teen-agers and young adults who are the future of his church.
The World Youth Day gathering here of an estimated 170,000 Catholics, ages 13 to 39, from about 70 countries is a biennial appeal to youth started by Pope John Paul eight years ago in Rome.
The other youth pilgrimages were to Argentina in 1987, Spain in 1989 and the pope's native Poland in 1991. Denver's is the first in what the church considers a "non-Catholic" country.
The rallies are part of the pope's "global strategy," in the words of a Vatican official, to reach the young with his old ideas.
And while he can still stir excitement and admiration among the faithful -- certainly among the T-shirted teen-agers with backpacks excitedly speaking a babel of languages as they converge on Denver -- many young Americans and their parents continue to discount the pontiff's conservative views on priestly celibacy, birth control and the place of women in the church.
An umbrella group of liberal Catholics -- including Dignity, an organization of homosexuals and their supporters, and Catholics for a Free Choice, which opposes the church's ban on abortions -- is holding an alternative meeting here during the pope's four-day visit.
Yesterday, the liberals pointed to a new Gallup Poll commissioned by them that suggests 90 percent of American Catholics under the age of 35 disagree with Pope John Paul on significant moral and social issues, including birth control.
The "respectful parallel agenda" of their convocation, a spokesman said, addresses the needs of "a church for the 21st century, one that courageously faces the problems and conflicts which alienate many people today."
The liberals' list of discussion subjects, which highlight their differences with the theologically conservative pope, included "women's rights, gay and lesbian rights, divorce and remarriage, direct election of pastors and bishops, sexuality, reproductive rights, the right to dissent, optional celibacy and ordination of women," the spokesman said.
While the denomination to which President Clinton belongs, the Southern Baptist Convention, has more in common with the pope on most of these issues than do the liberal Catholic groups, today's meeting between the pope and president is not likely to win any agreement on abortion.
Unlike the last two U.S. presidents with whom Pope John Paul met -- Ronald Reagan and George Bush -- President Clinton favors governmental guarantees of women's access to abortions, and this is known to have upset the Vatican.
On the plane Monday from Rome to Kingston, Jamaica, the first stop on his present trip, Pope John Paul was asked by reporters what he would say to Mr. Clinton today.
"I am an old pope, and you are a young president," was the reply.
When asked if he would discuss with the president their irreconcilable differences on abortion, the pope said, "We'll see."
Meanwhile yesterday, Catholic teen-agers continued to arrive at Denver's hotels and motels, guest rooms in countless private homes, a tent city and temporary lodgings at five high schools, a large parking garage and even the building of the National Western Livestock Show, where the showers normally are used to wash cattle.
Catholic Church officials and dozens of cheerful volunteers -- including a Southern Baptist woman who is an admirer of the pope -- have repeatedly stressed that this is a religious pilgrimage, not a convention, and pilgrims are supposed to make sacrifices.
The mood of the young pilgrims yesterday seemed mostly lighthearted, with country singer Wynonna Judd leading off the entertainment for an estimated 10,000 World Youth Day delegates in downtown Denver's Civic Center Park.
It has been temporarily renamed Celebration Plaza for this week's events.
And so many people want the honor of being seated at the altar for the pope's outdoor Mass Sunday that a stage the size of a football field is under construction in a state park south of here.