Pope Francis left Cuba on Tuesday for his first trip to the U.S., wrapping up a four-day visit to the communist island with an appeal to the people to rediscover their Catholic heritage and live a "revolution of tenderness."

Francis spoke at Mass at Cuba's holiest shrine, with President Raul Castro attending. His choice of words carried particular weight in a country whose 1959 revolution installed an atheist, communist government that sought to replace the church as the guiding force in people's live.

Around midday, he left for Washington for a visit in which he will meet with President Barack Obama, address Congress and the United Nations and take part in a Vatican-sponsored conference on families in Philadelphia.

The pontiff's homily in the Sanctuary of the Virgin of Charity of El Cobre was the latest in a series of carefully worded critiques of the Cuban system during his four-day visit.

At his Sunday Mass in Havana, he urged thousands of Cubans to serve one another and not an ideology. He also encouraged them to refrain from "looking to one side or the other to see what our neighbor is doing or not doing," words that resonated in a nation where the government controls most aspects of life.

Some 10 percent of Cubans regularly celebrate Mass, and the church has been trying to seize on the softening of the Cuban system under Raul Castro to rekindle the country's religious heritage.

"Generation after generation, day after day, we are asked to renew our faith," the pope said Tuesday. "We are asked to live the revolution of tenderness as Mary, our Mother of Charity, did."

The pope spoke in the foothills of the Sierra Maestra mountains where Castro and his brother Fidel commanded a guerrilla army that swept through the country and seized power in 1959.

After decades of official hostility to the church, the government has been gradually giving it space to operate in recent years, handing it back churches to reopen and allowing priests to run education programs and extensive outreach to the poor, sick and elderly.

As a result, Francis has been carefully balancing his desire to work with Castro's government on its path of internal reform and detente with the United States with his longstanding critiques of communism as a system that stifles the spirit.

His address to families and the symbolically potent flight to the United States underscore some of the big themes of his Cuba pilgrimage: encouraging reconciliation within families and between the U.S. and Cuba. He worked behind the scenes to broker the re-establishment of diplomatic relations between the two nations.

On his arrival in Cuba, the pope described the success of the negotiations as an example of peacemaking for the entire world. The Vatican spokesman said Monday night that this trip was aimed partly at encouraging progress in the continuing effort to normalize U.S.-Cuba ties in fields ranging from commerce to environmental cooperation.

Many Cubans believe in religious icons like the Virgin of Charity of Cobre, whose sanctuary is important to both observant Catholics and followers of Afro-Cuban Santeria traditions.

The foot-tall wooden statue is kept in an ivory-colored church with soaring red domes. One corner of the church where Francis celebrated Mass on Tuesday is dedicated to offerings left for the Virgin, including votives and thousands of handwritten notes.

"She means so much," said Juana Isabel Gonzalez Huart, a retired state worker. "She's the mother who protects us."

Francis, 78, appeared to be walking with some discomfort. He has needed extra help getting up and down stairs during this trip. Francis suffers from sciatica and has a bad knee.

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Associated Press writers E. Eduardo Castillo, and Anne-Marie Garcia in Havana and Andrea Rodriguez and Christine Armario in Holguin, Cuba, contributed to this report.

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Michael Weissenstein on Twitter: https://twitter.com/mweissenstein