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HAVANA, Cuba (CNN) — Celebrating Mass with thousands of Cubans on Monday, Pope Francis said that religion requires “missionary activity” and service, a subtle nod to the many restrictions his communist host country places on church activity.

The Pope urged Catholics “not to be satisfied with appearances or what is politically correct.” He also noted the “efforts and sacrifices” Cuban Catholics make to spread their faith, especially in remote areas of the country. The congregation for the Mass in Holguin, on Cuba’s northern coast, included Cuban president Raul Castro

Since landing in Cuba on Saturday, the Pope has largely avoided making overt political statements about Cuban politics.

“So far he’s sticking to the script of constructive engagement, with only mild, mostly veiled criticism, of the authoritarian regime,” said Andrew Chesnut, an expert on Latin America at Virginia Commonwealth University. “However, Pope Francis, the consummate diplomat, certainly spoke frankly with Raul Castro yesterday behind closed doors.”

Francis’ role in restoring diplomatic ties between the United States and Cuba could be one reason the Pope is avoiding the kind of criticism that his predecessors, St. John Paul II and Benedict XVI, made during visits to Cuba.

Cuban dissidents, though, have told reporters that they are unhappy that the Pope has not taken time to meet with them.

On a personal note, Monday is the anniversary of the day Pope Francis decided to become a Catholic priest in 1953.

Later on Monday, the Pope flies to Santiago, Cuba, where he will meet with Catholic clergy.

Trading gifts with Fidel Castro

On Sunday, Francis had a “friendly and informal” meeting with Fidel Castro at the former Cuban president’s residence, the Vatican said.

It lasted about 30 minutes, with the Pope and the communist leader exchanging books about religion. About 10 members of Castro’s family were present, according to the Vatican.

Castro, who is 89, rarely makes public appearances. He and Francis talked about the common problems of humanity, including environmental degradation, the Vatican said.

Castro’s son, Alex, photographed the meeting.

During a Mass in Havana’s Revolution Square earlier Sunday, the pope told a crowd estimated at 200,000 to “serve people, not ideas.”

“There is a kind of service which truly serves,” the Pope preached during his homily, “yet we need to be careful not to be tempted by another kind of service, a service which is self-serving.”

“There is a way to go about serving which is interested in only helping ‘my people,'” Francis continued, in remarks that some analysts interpreted as a criticism of Cuba’s communist government. “This service always leaves ‘your people’ outside, and gives rise to a process of exclusion.”

The Pope gets political

Since landing, Francis has made comments that many have considered carefully critical, and he has criticized Cuba in the past. In a 1998 book that he edited, he wrote that Cuba’s “authoritarian” and “corrupt” regime should be scrapped in favor of a representative democracy.

Francis is the third pope to visit Cuba, after John Paul in 1998 and Benedict in 2012. Francis’ call for Cuba to “open itself” to the world echoed John Paul’s remarks, a sign of the Catholic Church’s longstanding efforts to carve out space in a country that has been officially atheist for decades.

The Vatican has said the Pope believes religious freedom in Cuba means more than the ability to worship freely. It also entails the right to open schools and practice acts of charity, as well as other missions.

Raul Castro praises Pope

Raul Castro, who has said the Pope has inspired him to consider returning to the Catholic Church, effusively praised Francis on Saturday, particularly for his criticism of consumerism and environmental degradation. He also thanked him for his role in restoring ties between the United States and Cuba, while calling for the closing of the U.S. military base at Guantanamo Bay.

On Tuesday, the Pope will fly to Washington, a visible sign of the detente he has helped broker between Cuba and the United States.

“For some months now, we have witnessed an event which fills us with hope,” the Pope said. “The process of normalizing relations between two peoples following years of estrangement.”

Francis also called the restoration of diplomatic ties “an example of reconciliation for the entire world.”

Some American conservatives, however, have criticized the restoration of ties with the communist country, saying that the United States should have required the release of political dissidents.

U.S. President Barack Obama and Raul Castro spoke on the phone Friday to discuss the process of normalizing relations between the two countries.

Just last week, the United Nations announced that Castro would travel to New York to attend the U.N. General Assembly. It will be his first visit to the United States in more than half a century.

CNN’s Patrick Oppmann contributed to this report.