For Pope Francis, the restoration of relations between the US and Cuba has been a major diplomatic achievement, says the BBC's David Willey in Rome.
The US had imposed a trade embargo after Cuba's revolution, which it began to lift late last year.
After the 50-minute private audience on Sunday, Mr Castro told reporters: "The pontiff is a Jesuit, and I, in some way, am too. I studied at Jesuit schools."
After suggesting he might turn again to the Church, he added: "I mean what I say."
Both Mr Castro and his brother, revolutionary leader Fidel Castro, were baptised as Roman Catholics, but most Church activities were suppressed after the revolution.
The US had imposed a trade embargo after Cuba's revolution, which it began to lift late last year.
After the 50-minute private audience on Sunday, Mr Castro told reporters: "The pontiff is a Jesuit, and I, in some way, am too. I studied at Jesuit schools."
After suggesting he might turn again to the Church, he added: "I mean what I say."
Both Mr Castro and his brother, revolutionary leader Fidel Castro, were baptised as Roman Catholics, but most Church activities were suppressed after the revolution.
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