Pope Francis on Sunday delivered Mass to thousands of Cuban citizens in Havana before meeting with Fidel Castro, the former leader of the communist country.

The Argentine-born head of the Catholic Church commenced his first full day on the island country by offering an outdoor Mass at Revolution Plaza. President Raúl Castro -- who back in May promised that when the Pope came to Cuba he would go to all his Masses -- was in attendance.

According to the New York Times, Francis centered his homily on the idea of service, saying, “Whatever wishes to be great must serve others, not be served by others.”

At the close of the morning Mass, the Pope brought his liturgy toward the delicate work involved in keeping political peace, saying, “We do not have the right to allow ourselves yet another failure on this path of peace and reconciliation.”

After the mass the Pope went to the home of Fidel Castro for an informal visit, which lasted 40 minutes. The men exchanged books.

Diplomatic relations between the United States and Cuba are at the beginning stages of a thaw after over 50 years of icy relations. The Pope, who helped to set up meetings between Raúl Castro and Obama, is credited with doing much to help ease tensions between the two nations.

Andy Peraza Gonzalez, a Catholic who came out to hear the Pope, praised Francis for his political involvement. “I think he is a great man. A person who focuses on the real problems of the world. He’s been a great help to our country," he said.

New Jersey Gov. Chris Christie, a Catholic who is running for the Republican presidential nomination, spoke to CNN about Francis, suggesting the Pope may be overstepping his bounds by getting into politics. "I just think the Pope is wrong," he said, adding, "The fact is that his infallibility is on religious matters, not on political ones."

Watch the Mass: