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.

Pope Francis' cap flies off his head as he deplanes in Havana on September 19.

Pope Francis' cap flies off his head as he deplanes in Havana on September 19.

Cuban President Raul Castro walks with the Pope after his arrival on September 19.

Cuban President Raul Castro walks with the Pope after his arrival on September 19.

Children welcome the Pope shortly after he landed at the airport on September 19.

Children welcome the Pope shortly after he landed at the airport on September 19.

Soldiers march inside the airport in Havana during the arrival ceremony for the Pope on September 19.

Soldiers march inside the airport in Havana during the arrival ceremony for the Pope on September 19.

Crowds line the streets to greet the Pope in Havana on September 19.

Crowds line the streets to greet the Pope in Havana on September 19.

Pope Francis waves to people as he arrives at the Apostolic Nunciature in Havana on September 19.

Pope Francis waves to people as he arrives at the Apostolic Nunciature in Havana on September 19.

Women wave from an apartment building as Pope Francis travels from the airport on Saturday, September 19.

Women wave from an apartment building as Pope Francis travels from the airport on Saturday, September 19.

Worshippers in Havana gather before sunrise for the Pope's Mass on September 20.

Worshippers in Havana gather before sunrise for the Pope's Mass on September 20.

Crowds surround the Pope as he arrives in Revolution Square on September 20.

Crowds surround the Pope as he arrives in Revolution Square on September 20.

Pope Francis blesses the Virgin of Charity as he celebrates Mass on September 20.

Pope Francis blesses the Virgin of Charity as he celebrates Mass on September 20.

Pope Francis walks to the altar to celebrate Mass at Revolution Square in Havana on September 20.

Pope Francis walks to the altar to celebrate Mass at Revolution Square in Havana on September 20.

Pope Francis shakes hands with Fidel Castro in Havana on September 20. The Vatican described the 40-minute meeting at Castro's residence as informal, with an exchange of books.

Pope Francis shakes hands with Fidel Castro in Havana on September 20. The Vatican described the 40-minute meeting at Castro's residence as informal, with an exchange of books.

Pope Francis stands with Cuban President Raul Castro in the Revolution Palace in Havana, Cuba, on Sunday, September 20.

Pope Francis stands with Cuban President Raul Castro in the Revolution Palace in Havana, Cuba, on Sunday, September 20.

People cheer in Holguin as Pope Francis arrives for the Mass on September 21.

People cheer in Holguin as Pope Francis arrives for the Mass on September 21.

Security surrounds Pope Francis as makes his way to the Metropolitan Cathedral in Santiago, Cuba, on Tuesday, September 22. The Pope arrived in Cuba on Saturday and is scheduled to travel to the United States on Tuesday.

Security surrounds Pope Francis as makes his way to the Metropolitan Cathedral in Santiago, Cuba, on Tuesday, September 22. The Pope arrived in Cuba on Saturday and is scheduled to travel to the United States on Tuesday.

"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.

Pope Francis and Fidel Castro shake hands on Sunday afternoon.

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."

Programming Note Don't miss live coverage of Pope Francis' trips to Cuba and the U.S. on CNN and CNNgo, and see the people and places that shaped Pope Francis in a CNN special report, "The People's Pope," Tuesday at 9 p.m. ET.

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.