MIAMI, SEPT. 10 -- Pope John Paul II returned to the United States today for a second journey through the most diverse and independent of his Roman Catholic flocks. The essence of his 10-day pilgrimage -- the promise and the conflict, what he wants to tell America and what many Americans want to tell him -- was revealed or foreshadowed in the first hours of his arrival from Rome.

The pope was greeted at Miami's airport by President Reagan, who declared that "today all America applauds" the papal visit and urged the pontiff to speak his mind. "As you exhort us, we will listen with all our hearts," Reagan said. "We yearn to make this good land better still."

With his white cassock flapping in the hot breeze of this late-summer afternoon, the pope said he came as a pilgrim to a country of pilgrims, as a friend who already knew and loved America, and as a pastor whose mission was "to spell out once more the message of human dignity, with its inalienable human rights and its inevitable human duties."

Then, swiftly, he found himself immersed in many of the moral, religious and political issues that are unavoidable in this modern, upstart part of the world. As if to underscore that they would not surprise him, he had addressed many of those issues -- dissent from Catholic teachings, the church's relationship with Jews, homosexuality -- in a multilingual news conference aboard his chartered jet.

Protesters challenging the church's positions on women and homosexuality gathered outside St. Mary's Cathedral, the pope's first stop and site of his official welcome by the Miami diocese.

At St. Martha's Roman Catholic Church, Father Frank J. McNulty, a New Jersey priest chosen to represent his 53,000 American colleagues, told the pope with lyric eloquence about the difficulty with which they have learned to "hold fast to our Catholic value system while respecting the convictions of other people."

Citing a shortage of priests, McNulty urged the pope to "explore" the value of a celibate priesthood and the range of roles available to women in the church.

And the president, in a private session late this afternoon, sought the pope's help in implementing a peace plan for Nicaragua, a subject of special interest in Miami, with its large population of Cuban and Nicaraguan anticommunist exiles. In reply, the pope noted U.S. contributions to individual freedom and called upon the United States to share its wealth with poorer nations.

He had delivered that message as soon as his plane touched down. "I come as a friend of the poor and the sick and dying," he said on the tarmac, "those who are struggling with the problems of each day; those who are rising and falling and stumbling on the journey of life; those who are seeking and discovering, and not yet finding, the deep meaning of 'life, liberty and the pursuit of happiness.' "

The welcoming crowd at the airport included an estimated 1,500 people who endured 90-degree temperatures, high humidity and a brisk, damp wind that forced the pope to remove his skullcap.

Another 2,500 well-wishers waited outside St. Mary's Cathedral for the official diocesan welcome. For thousands more Miamians, the first look at the pope came during the parade from his meeting with Reagan at Vizcaya, a 10-acre Italian renaissance estate, to the home of Archbishop Edward A. McCarthy, where he spent the night.

But the crowds were far lighter than predicted, and officials were reluctant to estimate their size. "We probably overdid it in terms of cautioning people," Miami Mayor Xavier Suarez (R) said.

Most residents of this normally teeming city felt the pope's presence long before he arrived, even if they never saw him. Downtown Miami became a ghost town today as schools let out early, many businesses shut down and freeways -- normally closed in only one direction for a presidential motorcade -- were closed in both directions for president and pope.

The National Guard patrolled Biscayne Boulevard, one of the city's main arteries, outside the Hyatt Hotel where the Reagans rested briefly this morning. The extraordinary security was underscored by the presence here of Secret Service Director John Simpson, who rarely makes presidential trips.

"It was as tight as any security operation I've ever seen," one veteran Secret Service agent said.

Although police were prepared for as many as 250,000 people, the pope's most eager audience may have been one of his smallest: 750 priests, two from each diocese in the country, gathered at St. Martha's Roman Catholic Church late this afternoon to tell him, through McNulty, what it is to be a priest in 1987 in an America "that does not always appreciate the values that give our lives meaning."

"As we proclaim the moral message and help our people live out our value system," McNulty told him, "we sometimes find ourselves in tension . . . . Our church, and we as part of it, are committed to a bold proclamation of the truth, even when it weighs heavy or is countercultural. Yet we are also a forgiving church," too often perceived instead, he said, as "harsh, demanding."

"It saddens us that the church is not as credible to those within it, and to those outside of it, as we would like it to be."

McNulty said priests are worried by a severe shortage of priests. "Morale suffers when we see so few young men follow in our footsteps," he said. "Morale suffers when we see parishes without priests and prayer services taking the place of Sunday Mass." Despite the "generous young men" willing to give their lives to the Lord, he said, "the celibacy question -- as you so well know -- continues to surface. Its value has eroded and continues to erode in the minds of many."

He urged the pope to continue along "paths of support and exploration" on the issue of celibacy and on the ecclesiastical role of women "whose service . . . is essential for the life of the church. We would be encouraged if the Holy See would continue to explore the range of service that women might appropriately offer."

McNulty, observing that he had been urged to be honest and encouraging, was interrupted by applause 14 times, and the pope, in reply, only once. John Paul II did not answer their concerns specifically but emphasized their deep, personal relationship with Christ through sacrament and prayer.

A far more polyglot crowd awaits the pope Friday, the first full day of the 10-day tour that will take him to Columbia, S.C., and New Orleans later in the day and then to San Antonio, Phoenix, Los Angeles, Monterey, Calif., San Francisco and Detroit. From 200,000 to 500,000 people are expected at Tamiami Park on the western edge of the city at a 10 a.m. Mass.

Authorities here worried that continued heat would take a toll on worshipers, who may have to park as far as three miles from the site. The park was opened at midnight tonight in hopes that many would come early.