Parts of the film are very loosely based on real historical events concerning the ending of the Papacy of Paul VI, and the very short Papacy of John Paul I in 1978, and the collapse of the Banco Ambrosiano in 1982. Like the character Cardinal Lamberto, who becomes John Paul I, the historical John Paul I, Albino Luciani, reigned for only a very short time before being found dead in his bed with a just-completed report about the Jesuit order nearby.

Journalist David Yallop argues that Luciani was planning a reform of Vatican finances and that he died by poisoning; these claims are reflected in the film. Yallop also names as a suspect Archbishop Paul Marcinkus, who was the head of the Vatican bank, like the character Archbishop Gilday in the film. However, while Marcinkus was noted for his muscular physique and Chicago origins, Gilday is a mild Irishman.

The character of Frederick Keinszig, the Swiss banker who is murdered and left hanging under a bridge, mirrors the fate (and physical appearance) of Roberto Calvi, the Italian head of the Banco Ambrosiano who was found hanging under Blackfriars Bridge in London in 1982 (it was unclear until very recently whether it was a case of suicide or, as the Italian idiom has it, "being suicided." Courts in Italy have recently ruled the latter). The character of Licio Lucchesi, who moves between the church, organized crime and Italian politics, recalls Licio Gelli, head of the Propaganda Due Masonic lodge. The character of Joey Zasa bears many similarities to the flashy John Gotti.