After Meeting Fidel Castro, Pope Francis Preaches About 'Spirit of Poverty'

PHOTO: Pope Francis and Cubas Fidel Castro shakes hands, in Havana, Cuba, Sept. 20, 2015. PlayAlex Castro/AP Photo
WATCH Pope Meets With Cuban Leaders During Historic Visit

Pope Francis met with former Cuban leader Fidel Castro in Cuba today after this morning's Mass, and then delivered a sermon in which he praised the "spirit of poverty" and warned against wealth.

At a vespers celebration with priests, nuns, and seminarians gathered at Havana Cathedral, the pope went off his prepared text and spoke spontaneously, warning against letting riches rule your life and calling poverty "the wall and the mother of consecrated life."

"The mother, because it created more trust in god, and the wall because it protected it from worldliness," Francis said. "How many souls, like the one of the sad young man, who started off well, but who then became worldly, and ended mediocre, without love, because richness impoverishes. It makes us poor in the only wealth that is worth having, to put our safety in other things. The spirit of poverty, of letting go of everything to follow Jesus. This is in the Gospel several times. The first ones left the boats, and the nets, and followed him. They left it all to follow Jesus."

Later, he encouraged a group of youth at the Centro Cultural Padre Felix Varela to "dream big."

"Open yourself, dream big, don’t strike yourself," he said. "Open yourself, and dream. Dream that the world with you, can be different. Dream that if you give the best of yourself, you’ll help make the world a different place. Don’t forget. Dream. Sometimes you dream too much and life cuts your path. It doesn’t matter, dream, and share your dream, make it big, because bigger the ability to dream, even if life leaves you half way to it, you would have walked a longer path."

In his meeting with the former Cuban leader, Francis gave Castro several books, as well as a book and two CDs of homilies and a copy of his two encyclicals.

In return, Castro gave Francis an interview book entitled, "Fidel and Religion," written in 1985 by Freitas Betto. Fidel wrote a dedication in it: "For Pope Francis, on occasion of his visit to Cuba, with the admiration and respect of the Cuban people."

The atmosphere was "familiar and informal," Father Lombardi said, and the conversation centered around "protecting the environment and the great problems of the contemporary world."

PHOTO: Pope Francis arrives at Revolution Square in Havana, Cuba, Sept. 20, 2015. RODRIGO ARANGUA/AFP/Getty Images
Pope Francis arrives at Revolution Square in Havana, Cuba, Sept. 20, 2015.

Francis began a busy day in Cuba by holding Mass this morning in Havana's Revolution Square.

When Francis drove by in his open-sided Peugeot popemobile, the crowds sang and waved Cuban, Vatican and Argentine flags.

As he headed towards the square, he passed by two men and a woman who tried to seize the moment. They approached with leaflets, threw them toward the pontiff and were instantly pounced on by security guards. Once the three were detained, the guards picked up the fliers.

PHOTO: Pope Francis arrives for Mass at Revolution Plaza in Havana, Cuba, Sept. 20, 2015. Ramon Espinosa/AP Photo
Pope Francis arrives for Mass at Revolution Plaza in Havana, Cuba, Sept. 20, 2015.

Francis then began Mass, which was attended by 300,000, according to the Cuban government.

"God’s holy and faithful people in Cuba is a people with a taste for parties, for friendship, for beautiful things," Francis said. "It is a people which marches with songs of praise. It is a people which has its wounds, like every other people, yet knows how to stand up with open arms, to keep walking in hope, because it has a vocation of grandeur."

PHOTO: Pope Francis (C) walks to the altar to give mass at Revolution Square in Havana, Cuba, Sept. 20, 2015. FILIPPO MONTEFORTE/AFP/Getty Images
Pope Francis (C) walks to the altar to give mass at Revolution Square in Havana, Cuba, Sept. 20, 2015.

"Today I ask you to care for this vocation of yours, to care for these gifts which God has given you, but above all I invite you to care for and be at the service of the frailty of your brothers and sisters. Do not neglect them for plans which can be seductive, but are unconcerned about the face of the person beside you. We know, we are witnesses of the incomparable power of the resurrection, which everywhere calls forth the seeds of a new world."

Francis also gave several children their first communion.

Francis' day continued with a formal meeting with Cuban President Raul Castro.

Raul Castro gave the pontiff a large crucifix made by a Cuban artist. The pope gave the president a painting of the patron saint of Cuba, La Virgen de la Caridad del Cobre. The pope will visit the shrine where the statue of the La Virgen is kept Monday evening.

After his first meeting with Fidel Castro, the pope then went to back to the Nunciature, and then with a small group, he went to Fidel Castro's home for another meeting. Castro’s wife was also there with his children and grandchildren. They were together for about 30 to 40 minutes.

Francis began his 10-day trip to Cuba and the U.S. Saturday, addressing the crowd at Jose Marti International Airport upon his arrival and hailing the two countries for reestablishing diplomatic relations following a long period of estrangement.

PHOTO: Pope Francis hugs a boy as Cubas President Raul Castro looks on after landing at the airport in Havana, Cuba, Saturday, Sept. 19, 2015.Ismael Francisco/Cubadebate Via AP
Pope Francis hugs a boy as Cuba's President Raul Castro looks on after landing at the airport in Havana, Cuba, Saturday, Sept. 19, 2015.

Get real-time updates as this story unfolds. To start, just "star" this story in ABC News' phone app. Download ABC News for iPhone here or ABC News for Android here.

The Associated Press contributed to this report.