Lamberto was a Sicilian male who would become a Cardinal by 1979. By then it was obvious that Pope Paul VI was in very poor health. Realizing that the Vatican Bank was being used for corrupt ends, Michael Corleone approached Lamberto to warn him of what was happening, believing that he was in a position to do something about the situation.
Corleone felt an immediate connection to the Cardinal, and under his gentle prodding made his first confession in three decades. Hearing the depths of Corleone's sins - including the murder of his brother Fredo - Lamberto told him that it was just that he suffer. Even though he believed Corleone would not turn from his ways he absolved him of his sins.
Paul VI soon died. In the conclave that followed Lamberto was elected Pope. Praying he asked God why his brother Cardinals would elect him Pope, before accepting the election. Lamberto decided to take the names of his two immediate predecessors, taking the name John Paul I.
To the consternation of Archbishop Gilday and Vatican Bank chief account Keinszig, the new Pope had very different ideas about how things were to be run and immediately ordered an investigation into the bank. Keinszig skipped town with a large amount of money, intensifying the Pope's suspiscions about the bank. Fearing exposure the men in charge decided to murder John Paul I. A short time later the Pope was found dead in his bed by a nun after consuming poisoned tea.
The character of Pope John Paul I seen in The Godfather Part III was loosely based on the real John Paul I. The real John Paul I was a northern Italian male named Albino Luciani, who had previously served as Patriarch of Venice. He was elected in 1978 to the Papacy following the death of Paul VI. At 33 days he had one of the shortest reigns in Papal history, with some people speculating that he had been murdered.