Cuban President Raul Castro, a staunch Marxist, met with Pope Francis at the Vatican today (Sunday, May 10), thanking the pontiff for his role in brokering an historic agreement between Havana and Washington, ending more than 50 years of animosity.

"Raul Castro thanked the Pope for his mediation between Cuba and the United States," said Vatican spokesman Federico Lombardi of the exchange that also focused on Francis' upcoming visit to Cuba.

The first South American Pope played a key role in the secret negotiations between the United States and Cuba, which led to the surprise announcement in December that the two countries would seek to restore diplomatic ties after more than 50 years of tensions. Last year when President Obama visited Vatican Pope Francis strongly demanded an end to American embargoes on Cuba and asked him to restore friendship between Cuba and the U.S.  Soon after that visit , in December 2014 , Obama ordered lifting travel bans and trade bans against Cuba as the first sign of restoring diplomatic relationship with Cuban people.

During the meeting, Castro offered the pontiff a painting by Cuban artist Kcho inspired by the plight of illegal immigrants stranded at sea -- a subject close to the Pope's heart. Pope Francis in turn presented Castro a medal of Saint Martin de Tours, a French saint celebrated for having given his coat to a beggar, and urged others to "clothe and support the poor."

Raul Castro, who was accompanied by his Foreign Minister Bruno Rodriguez Parrilla, held a private hour-long meeting with Pope Francis. in a small room adjoining the Paul VI Audience Hall.

Castro said he was "very struck" by the Catholic leader's "modesty and wisdom" and promised to attend all the masses given by Francis on his Cuba visit in September. "I read all the Holy Father's speeches," Castro said, adding that if the pope "continues to speak in this way, one day I will start praying again and return to the Catholic Church. And I'm not saying that as a joke."

Pope Francis would visit the Caribbean island in September 2-15, becoming only the third pontiff to do so after John Paul II in 1998 and Benedict XVI in 2012. Jorge Bergoglio, then auxiliary bishop of Buenos Aires and now Pope Francis, accompanied John Paul II  (in 1998) on the first papal visit to Cuba, during which John Paul II called for Havana to be brought in from the cold war. "Let Cuba open itself to the world, and let the world open itself to Cuba,"  said Late Pope John Paul II (who is now Saint John Paul II). In 2012, Pope Benedict XVI also visited Cuba and met Fidel Castro, the long-term president and elder brother of Raul Castro.

The Catholic Church has consistently backed calls for the lifting of the US trade embargo against Cuba, while staunchly supporting Cuban Catholics and pressuring Havana to release political prisoners, many of whom are Catholic activists.

The Vatican's mediation between Cuba and the U.S. administration was a diplomatic success for the Holy See and had a considerable impact in the mainly Catholic Latin America.

Cuba's President Raul Castro (L)during a meeting with US President Barack Obama on the sidelines of the Summit of the Americas at the ATLAPA Convention center in Panama City on April 11, 2015 (AFP Photo/Mandel Ngan)

Picture released by the Vatican Press Office shows Pope Benedict XVI with Cuban leader Fidel Castro (R) during a meeting in Havana on March 28, 2012 (AFP Photo/)

Cuban president Fidel Castro (R) listens to Late Pope Saint John Paul II on the tarmac of the Jose Marti International Airport in Havana January 21, 1998 (AFP Photo/Michel Gangne)